Precision in harsh environments

Abstract

Microsystems are increasingly being applied in harsh and/or inaccessible environments, but many markets expect the same level of functionality for long periods of time. Harsh environments cover areas that can be subjected to high temperature, (bio)-chemical and mechanical disturbances, electromagnetic noise, radiation, or high vacuum. In the field of actuators, the devices must maintain stringent accuracy specifications for displacement, force, and response times, among others. These new requirements present additional challenges in the compensation for or elimination of cross-sensitivities. Many state-of-the-art precision devices lose their precision and reliability when exposed to harsh environments. It is also important that advanced sensor and actuator systems maintain maximum autonomy such that the devices can operate independently with low maintenance. The next-generation microsystems will be deployed in remote and/or inaccessible and harsh environments that present many challenges to sensor design, materials, device functionality, and packaging. All of these aspects of integrated sensors and actuator microsystems require a multidisciplinary approach to overcome these challenges. The main areas of importance are in the fields of materials science, micro/nano-fabrication technology, device design, circuitry and systems, (first-level) packaging, and measurement strategy. This study examines the challenges presented by harsh environments and investigates the required approaches. Examples of successful devices are also given.

OPEN

Microsystems & Nanoengineering (2016) 2, 16048; doi:10.1038/micronano.2016.48

www.nature.com/micronano

REVIEW ARTICLE

Precision in harsh environments

Paddy French1, Gijs Krijnen2 and Fred Roozeboom3,4

Microsystems are increasingly being applied in harsh and/or inaccessible environments, but many markets expect the same level of functionality for long periods of time. Harsh environments cover areas that can be subjected to high temperature, (bio)-chemical and mechanical disturbances, electromagnetic noise, radiation, or high vacuum. In the eld of actuators, the devices must maintain stringent accuracy specications for displacement, force, and response times, among others. These new requirements present additional challenges in the compensation for or elimination of cross-sensitivities. Many state-of-the-art precision devices lose their precision and reliability when exposed to harsh environments. It is also important that advanced sensor and actuator systems maintain maximum autonomy such that the devices can operate independently with low maintenance. The next-generation microsystems will be deployed in remote and/or inaccessible and harsh environments that present many challenges to sensor design, materials, device functionality, and packaging. All of these aspects of integrated sensors and actuator microsystems require a multidisciplinary approach to overcome these challenges. The main areas of importance are in the elds of materials science, micro/nano-fabrication technology, device design, circuitry and systems, (rst-level) packaging, and measurement strategy. This study examines the challenges presented by harsh environments and investigates the required approaches. Examples of successful devices are also given.

Keywords: atomic layer deposition; harsh environments; micro/nano-fabrication technology; packaging; sensors

Microsystems & Nanoengineering (2016) 2, 16048; doi:10.1038/micronano.2016.48; Published online: 10 October 2016

INTRODUCTIONThe earliest known weights and measures systems date to approximately the 4th or 3rd millennium BC and was developed among the ancient peoples of Egypt, Mesopotamia, the Indus Valley, and perhaps Elam (Iran). Over the centuries, different weights and measures systems have emerged. Sensors were designed that could use these systems to measure and dene a wide range of parameters. With the industrial revolution, the need for measurements increased greatly, and measurement devices were increasingly used in harsh environments. In the early days, sensors were quite simple and relied on the operator to control the machinery. With the advent of automation, it became increasingly necessary to use sophisticated sensors as components of a control system. These sensors were designed to simply control the machinery. In the automotive industry, this approach was used to improve performance. In recent years, an increasing need has arisen for sensors for environmental control, for example, regulation of the exhaust from automobiles or from the process industry. The source of the harshness might not always be the measurand itself, for example, in the case of measuring pressure at high temperature.

This study denes a harsh environment as any environment that can impede the operation of the device. Harshness can originate from various sources, and examples include the following:

High pressure

High temperature

Shock/high vibration

Radiation

Harsh chemicals

Humidity

Biological (including inside the body for medical implants).

Many industries must address these environments, and selected examples are given in Table 1. The extent of the harshness and the limitations on design differ from application to application.

The approach chosen depends on the domain of the harshness and the application. Some examples are given in Table 2.

Table 2 shows a matrix of the harsh conditions (columns) and hierarchical levels at which the specic harsh conditions can be counteracted (rows). The number of + signs indicates the appropriateness of the given level for improved resistance against harshness. For example, chemical harsh environments can best be counteracted by proper material choices and packaging as well as the choice of fabrication technology. Typical approaches to improving resistance to harsh environments are listed below:

Materials

J Chemically inert

J High glass or melting temperature

J Dense materials to reduce device exposure to radiation

J Material combinations and alloying

1Faculty of Electrical Engineering, Mathematics and Computer Science, TU Delft, Delft, The Netherlands; 2Faculty of Electrical Engineering, Mathematics and Computer Science, Twente University, Enschede, The Netherlands; 3Eindhoven University of Technology, Department of Applied Physics, PO Box 513 5600 MB, Eindhoven, The Netherlands and

4TNO Holst Centre, High Tech Campus 21, 5656 AE, Eindhoven, The Netherlands. Correspondence: Paddy French ([email protected])

Received: 3 February 2016; revised: 27 May 2016; accepted: 19 June 2016

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Technology

J Fabrication method, conditions, annealing

J Additional layers (for example, to prevent delamination and increase resilience), additives

Device design

J Special zones to absorb mechanical/chemical loading or thermal cycling

J Choice of measurand (for example, a derivative quantity)

Packaging

J Special zones to absorb mechanical/chemical loading or thermal cycling

J Materials used in packaging (for example, chemically inert)

System

J Isolation of the electronics from the harsh environment (for example, high temperature)

J Modes of operation (for example, intermittent to prevent self-heating or low power)

As an example, in certain cases, the best approach might be optical (that is, often non-invasive) sensing. This method can allow remote sensing in applications where the sensing system is not required to be exposed to the harsh environment, for example, excitation by a laser and measurement of the optical absorption or scattering in the area of interest1 or use of ber materials that can withstand high temperatures, such as sapphire2. Optical signals in bers are also not affected by electrical noise3. Coating layers, such as those used with atomic layer deposition (ALD)4,5, may

protect the device from harsh chemical environments as well as the use of chemically inert materials such as SiC6,7. For high-

temperature applications, wide band gap materials such as SiC, GaN, AlN or diamond810 may be used. More recently, interest in the use of graphene has increased for high temperature applications11.

The following sections examine the sources of harshness and the approaches required to address them and allow accurate measurement.

PROCESSING AND MATERIALSSubstrate materialsStandard electronics are fabricated using single-crystalline silicon, which is an excellent material from both electrical and mechanical perspectives. However, silicon encounters limitations with respect to temperature range. Silicon-based electronic devices are not usually operated above 150 C owing to junction leakage. Above ~ 200 C, the material becomes intrinsic owing to thermal generation of electronhole pairs. This effect can be offset locally using the exclusion principle12. An alternative approach is to use a semiconductor with a wider band gap, such as SiC, GaN, and AlN. However, the Integrated Circuit (IC) technology for these materials is less well developed, and the substrates are more expensive.

Thin-lm depositionModern semiconductor thin-lm deposition techniques are treated in detail in textbooks13. A non-exhaustive list of current thin-lm deposition methods is shown in Table 3 together with short descriptions.

General selection criteria for thin-lm deposition are given in Table 4.

Brief comparison of the most popular thin-lm technologies For over 50 years, the most popular thin-lm deposition methods in industry have been physical vapour deposition (PVD) and chemical vapour deposition (CVD), with atomic layer deposition (ALD) emerging in applications where nanometer-size layer thicknesses or pinhole free layers are crucial. In particular, spatial ALD has potential as an interesting newcomer14,15. On the basis of

the selection criteria listed above, one can compare the different deposition methods compiled in Table 5.

CVD This method is widely used in the deposition of thin lms and is available in three forms: atmospheric-pressure CVD (APCVD), low-pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). The main layers deposited are silicon oxide, silicon nitride, silicon and silicon carbide (SiC). For harsh environments, SiC is of particular interest because it is chemically inert and can operate at high temperatures. Using PECVD, SiC can be deposited at temperatures o400 C, thus making it suitable as a coating layer for an IC intended for use in harsh chemical environments. This layer has been observed to be pinhole-free for thicknesses greater than ~ 200 nm and can be used as the mechanical material for pressure sensors16. LPCVD SiC can also be used, but this material has a deposition temperature above 700 C and thus cannot be used as a coating layer for standard circuitry (unless an alternative metal layer is used). Other wide band gap semiconductors such as GaN and AlN are also useful materials for high temperature applications.

ALD This technique is used to deposit lms in an atomic layer-by-layer manner4,5, which enables the deposition of notably thin pinhole-free layers of high quality. ALD was rst published under the name molecular layering in the early 1960s by Professor

Table 1 Harsh environments in different industries

Industry Harsh environment

Oil/process Temperature, chemical, pressureAutomotive Temperature, chemical, pressure, electric elds Space Temperature swings, radiation, high vacuum Aircraft Temperature, pressure, vibration, electric elds Farming Pesticides, chemical, biologicalMedical implants Biological (for example, stomach, colon, blood)

Table 2 Examples of harsh environments and domains that offer the most effective approaches

Chemical Thermal Mechanical EM loading* Radiation

Materials ++ ++ + + Technology + ++ +Device design + ++ +Packaging ++ + ++ ++ + System + + + +

*EM loading occurs when electromagnetic disturbances are present. EM: electromagnetic.

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Table 3 Concise survey of mainstay thin-lm deposition methods and characteristics

ElectroplatingFilm formation from chemicals in dissolved electrolytic solution placed onto substrate surface with a seed layer on top that is electrically biased.

Spin coatingFilm formation from chemical reaction between liquid-phase sources (often solgel) that have been applied onto the rotating substrate and subsequently heated.

Thermal oxidationFilm formation by chemical oxidation of the substrate surface.

Physical vapor deposition (PVD)Film formation by condensation of gasied source material directly transported from source to substrate through the gas phase via

Evaporation (thermal, E-beam)

Molecular beam epitaxy (MBE)Pulsed laser deposition (PLD)Reactive PVDSputtering (DC, DC magnetron, RF)

Chemical vapor deposition (CVD)Film formation by chemical reaction between mixed gaseous source materials on a substrate surface using:

Atmospheric-pressure CVD (APCVD)

Low-pressure CVD (LPCVD)Plasma-enhanced CVD (PECVD)Metal-organic CVD (MOCVD)

Atomic layer deposition (ALD)A sub-class of CVD with lm formation via sequential cycling of self-limiting chemical half-reactions on the substrate surface. Each reaction cycle accounts for the deposition of a (sub)monolayer. The reaction can be activated by thermal energy or plasma enhancement or can be performed in the spatially divided regime14.

Thermal ALD Plasma-enhanced ALD (PEALD)Spatial ALD (S-ALD)

Table 4 Criteria for selection of thin-lm techniques

Deposition rate Deposition directionality

Directional: suitable for lift-off, 3D feature lling Non-directional: suitable for step coverage

Film thickness and composition uniformityWithin-waferWafer-to-wafer (single-wafer deposition) and run-to-run (batch deposition)

Film quality (physical and chemical properties)AdhesionBreakdown voltageFilm density, pinhole densityGrain size, grain boundary features, and grain orientation Impurity levelStoichiometryStress and yield strengthHermeticity

Functional materials to be depositedMetals/conductorsDielectrics, magnetics, piezoelectrics, etc.Polymers

Cost-of-ownership and operation cost

Abbreviation: 3D, three dimensional.

Koltsov from Leningrad Technological Institute, although the basic concept was proposed by Professor Aleskovski in his PhD thesis in 1952. The characteristic feature that distinguishes ALD from other deposition technologies such as CVD, PVD, solgel synthesis, and spray pyrolysis is the self-limiting chemisorption of precursors in each half-cycle5. This feature makes ALD unique in sub-nanometer

lm thickness and conformality control, offering next to (nearly) equal growth-per-cycle values for identical precursors in different equipment17. The remaining drawback of conventional ALD that prevents it from cost-effective commercial use is the low deposition rate, but this shortcoming has been largely overcome by the launch of spatial atmospheric ALD5.

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Dielectriclayers,insulatinglayers,andsoon,

solarcellsurfacepassivation18

MetalALDusingthermal

chemistry

Conductivepathways,catalyticsurfaces,MOS

devices19

modication,creationofcomposites,diffusion

barriers,andsoon4

Depositionofprotectiveandinsulating

coatings,opticalandmechanicalproperty

modication,formationofcomposite

structures,conductivemedia

Plasma-orradical-

DRAMstructures,MOSFETandsemiconductor

devices,capacitors19

Cu-andLi-diffusionbarrierlayers15,21

DirectionalityUniformityFilmdensityGrainsize(nm)ImpuritylevelCost

50100120YesPoorPoor10100HighVerylow

5010010100YesPoorPoor10100LowHigh

TypeofALDTemperaturerangeViablelayersPrecursors/reactantsApplications

CatalyticALD432CwithLewisbase

catalyst4

SomedegreeVerygoodGood20LowHigh

PECVDMainlydielectrics20030010100SomedegreeGoodGood10100VerylowVeryhigh

LPCVDMainlydielectrics6001200Metal100

IsotropicVerygoodExcellent110VerylowVeryhigh

ALD(thermal)Mainlydielectrics503000.11Isotropicstep

conformal

SuperiorSuperior110VerylowVeryhigh

ALD(plasma)Mainlydielectrics202000.11IsotropicSuperiorSuperior110VerylowVeryhigh

ALD(spatial)Mainlydielectric20200110Isotropicstep

conformal

SuperiorSuperior110VerylowHigh

4 TiCl 4 ,ZrCl 4 , H 2 O(Ref.4)Highk-dielectriclayers,protectivelayers,anti-

reectivelayers,andsoon4

Al 2O 3,ALD30300CAl 2O 3,metaloxides

M(C 5H 5) 2,(CH 3C 5H 4)M(CH 3) 3,Cu(thd) 2,Pd(hfac) 2,

Ni(acac) 2, H 2(Ref.20)

Al(CH 3) 3,Ti(OiPr) 4, (C 2H 5) 2Zn,/H 2O(Ref.18)Polymersurfacefunctionalizationand

Abbreviations:ALD,atomiclayerdeposition;CVD,chemicalvapordeposition;LPCVD,low-pressureCVD;PECVD,plasma-enhancedCVD.

Table6ExamplesofreactionmechanismsandapplicationsforALDlayers

BN,ZrO 2,CNTs,polymerparticlesVariousgases:useofrotaryuidbedreactoris

highlyimportanttoensureuidationof

particles4,22

Organometallics,MH 2Cl 2,terbutylimidotris

(diethylamido)tantalumTBTDET),bis

(ethylcyclopentadienyl)ruthenium,NH 3(Ref.4)

Table5Comparisonofthin-lmdepositionmethods

19 Trimethylaluminum,TiCl 4 , H 2 O,Ti(OiPr) 4 ,

(Metal)(Et) 2(Ref.4)

ALDonpolymers25100C(Ref.4)Al 2O 3, ZnO, TiO 2,andmetaloxidesonfor

example,PET,PENpolyimideforexible

200Metal200

450800C(Ref.4)Puremetals(thatis,Ta,Ti,Si,Ge,Ru,Pt)20 ,

andmetalnitrides(thatis,TiN,TaN,and

soon)4,22,23

Depositionrate

(s1 )

Dielectrics110

Dielectrics110

Metaloxides(thatis,TiO 2,ZrO 2)

solarcellsanddisplays14 andontextile15

temperature(o C)

Abbreviation:ALD,atomiclayerdeposition;MOS,metaloxidesemiconductor.PET,Polyethyleneterephthalate;PEN,Polyethylenenaphthalate.

ProcessMaterialSubstrate

175400C(Ref.19)Metaluorides,organometallics,catalytic

metals19

particles,100400Cfor

metal/alloyparticles4

ThermalevaporationMetalorlow-melting

pointmaterials

E-beamevaporationBothmetaland

dielectric

SputteringBothmetaland

dielectric

ALDonparticles25100Cforpolymer

enhancedALDforsingle-

andmultiple-elementALD

conductors

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Selected examples of ALD processes and applications are given in Table 6, which is based on Refs. 4,5,15,1823.

The high quality of the layer makes it highly suitable for biomedical applications4 and for the use in harsh chemical environments in which the underlying layers must be protected 24,25.

Graphene This material is an sp2-hybridized allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex. The material is 200 times stronger than steel by weight and has high thermal and electrical conductance. Graphene has a tensile strength of 130 GPa and a Youngs modulus of 1 TPa, is able to operate at high temperature, is suitable for harsh chemical environments and is acceptable as a coating layer in medical implants26.

Polymers Polymers are composed of a large, chain-like molecular structure made of monomers, which are covalently linked in three-dimensional networks. There are many examples of both natural and synthetic polymers and polymers have found numerous applications in the sensor eld. One of these applications is in medicine where a number of polymers have been found to be suitable for both in vitro and in vivo devices. These polymers do not generate any bodily reactions and are also able to withstand the biological environment. Examples of these polymers include the following:

Natural

J Plants: cellulose, natural rubber

J Animals: collagen, heparin, DNA

Synthetic

J Parylene

J Silicone rubber

J Polyethylene

J Polypropylene

J Polymethyl methacrylate (PMMA)

J Polyvinyl chloride

J Polyether ether ketone (PEEK)

Parylene is a trade name for a variety of chemical vapor-deposited poly(p-xylylene) polymers used as moisture and dielectric barriers. The most common of these materials is parylene-C, which was rst discovered by Szwarc in 1947 and is suitable for a range of applications, including medical implants2732. Parylene is deposited by selecting a raw dimer of the material and heating it to nearly 150 C. The vapor is pulled out under vacuum and heated to high temperature, which allows for sublimation and splitting of the molecule into a monomer. This monomer is subsequently further drawn by vacuum for deposition onto the required substrate.

Silicone rubber is well known for its elastic behavior with a maximum elongation that can range from 90 to 900% depending on the composition33. This material can form watertight seals and is water-repellent, although rather permeable to gas, which is an interesting combination for medical applications. Combination with liquid metals allows for use in strain sensing with up to 50% change in resistance and application, for example, in smart gloves34,35 and soft articial skin36.

PMMA is probably better known under the name plexiglas and is a low-cost, optically transparent material that has numerous applications in sensing. PMMA is often used in the formation of microuidic chips37, for example, in the form of molded structures.

PEEK is a semi-crystalline plastic used abundantly in engineering components owing to its mechanical strength, high wear resistance, chemical inertness in organic, and aqueous

environments, high maximum operating temperature (up to 250 C) and low thermal degradation. Although PEEK has relatively high glass transition and melting temperatures, it can be processed by molding and extrusion, computer numeric controlled machining38, and recently even through additive manufacturing by either fused deposition modeling (FDM)39 or selective laser sintering40. The properties of PEEK make it a biocompatible material that is used in the reconstruction of large cranial osseous defects41, among other applications.

Ceramics Ceramics span a wide range of inorganic materials that can be highly ordered or amorphous (for example, glass). Ceramics such as sintered alumina are thermally and chemically resistant and can withstand temperatures as high as 42000 C (Ref. 42) and aggressive applications in, for example, oven linings and plasma-etching equipment43. Some ceramics are also piezoelectrics and can therefore be applied in both sensors and microactuators. A review of the development of piezoelectric ceramics can be found in Ref. 44. In terms of medical applications, ceramics have been used in dental and orthopedic implants. For these applications, ceramics are robust and biocompatible (that is, they do not generate any reactions from the body). Ceramics have replaced many metal devices for implants45, and examples exist of biodegradable ceramics for bone implants46. Glass is used in the encapsulation of implants.

HARSH ENVIRONMENTSThis section discusses the different types of harsh environments.

In certain cases, attempts are made to measure parameters such as high temperature or alternative parameters at high temperature. There are many such examples within different signal domains.

High temperatureSilicon is an excellent material for sensors but has certain limitations in terms of its process temperature window (the temperature tolerance range in which device fabrication can take place) and device-operating temperature range.

If the device uses a pn junction, the temperature is limited to ~ 125 C. Above this temperature, the junction suffers from leakage. Near 200 C, extrinsic silicon is reverted to intrinsic silicon owing to the high generation of electronhole pairs. This transition can be extended to higher temperatures for small volumes of silicon (which might contain the device) using the exclusion principle12. The device is designed and biased such that a small contact must supply a large number of minority carriers, which means that if the semiconductor begins to generate large numbers of electron-hole pairs, the minority carriers are immediately absorbed and the volume remains an extrinsic semiconductor. If we are able to fabricate devices within this region, they can continue to operate normally at temperatures above the transition. In this work12, Hall devices were fabricated to operate at up to 500 C. The basic principle is shown in Figure 1.

For direct temperature measurements, platinum resistors can be a good option. Platinum is stable up to ~ 1000 C (Ref. 47), and platinum-based thermocouples are widely used in temperature measurement. The materials used in the construction determine the device sensitivity and also the temperature range. Certain thermocouples are able to measure temperatures up to 3000 C (Ref. 48). These high-temperature measurements require refractory (that is, high melting point) materials such as tungsten and rhenium.

In applications such as pressure sensing at high temperature, the sensor must be exposed to the environment. One option is to use wide band gap semiconductor materials that maintain their electrical properties up to higher temperatures. For example, SiC

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power devices have been operated successfully at temperatures of ~ 600 C. Diamond devices have been found to operate at even higher temperatures, and Schottky devices have operated at 1000 C (Ref. 49). Ceramic materials and metals such as tungsten maintain their mechanical properties at notably high temperatures (41000 C). These alternative materials can be combined with device/package designs that protect the sensing element from the high environmental temperatures. This approach is illustrated in Figure 2. The use of a silicon membrane in direct contact with the environment limits the working temperature for measuring pressure. If pn junctions are used, the temperature is limited to below 150 C. Above 500 C, the good mechanical properties are lost, and the materials become plastically deformable50. The temperature range can be extended using a metal membrane with a mechanical connection to the chip. In this case, the pressure can be transduced to a pressure sensor via an incompressible uid or through a hard contact to a stress sensor. Using the same basic principle, the sensor chip can be further isolated from the environment by adding a longer mechanical element that allows for further thermal isolation, as shown in the top right option, which greatly improves the temperature range, although it also increases the costs.

The use of SOI wafers5153 or polysilicon piezoresistors54 can also increase the temperature range because these materials do not rely on pn junctions. Further improvement can be achieved by selecting a wider bandgap material such as SiC or SiC on an insulator. All of these semiconductors have limitations in terms of their temperature range and bursting pressure. If a metal or other material is used as the membrane (Figure 2; Ref. 55), the material

and thickness of the membrane can be chosen to meet the pressure and temperature requirements.

An alternative approach to high temperature application is the use of optical bers. In the oil industry, for example, in the drilling of holes and pipes, long distances often lie between the point of measurement, and the readout in addition to high temperature and high electrical noise in the environment. In this case, optical bers offer the best option and bers capable of withstanding high temperatures can be used56. Alternatively, free-space optics can be used57. In this case, the light source and the receiver are isolated by distance from the harsh environment, and only the light passes through the device.

With ber-optic devices, the temperature sensitive components of the system can be sheltered from high ambient temperatures. The measurement process can use interference58 or FabryProt interferometry59. An example of an optical-ber pressure sensor is given in Figure 3 (Ref. 60).

In extraction and recovery in the oil industry, the point of measurement might be located 41 km from the instrumentation.

Furthermore, the point of measurement and the path to the instrumentation may be at high temperature and also may be subjected to high electrical interference. An optical system can address many of these problems, and one such example is given in Figure 4. The use of optical techniques offers many opportunities for operation at high temperatures61.

The operating temperature range of standard CMOS can be increased by careful consideration of the design62,63. Through

careful design of the preamplier, the system was able to operate at temperatures up to 275 C.

Low temperatureExtremely low temperatures (o100 K) also create challenges for sensors and the electronics. Fibre Bragg gratings have been shown to work effectively at extremely low temperatures64, as

have a number of other devices65,66. At moderately low

temperatures, approximately 50 C, problems occur with the packaging and the batteries in devices. As the temperature decreases further to cryogenic temperatures and below, the challenges greatly increase. Silicon continues to operate, but adjustments to the processing and the design are required. An overview of the issues for operation at cryogenic temperatures and below is given in Ref. 67. Enhanced operation at these low temperatures using high-K dielectrics can aid in maintaining correct operation68.

Oxide

Extrinsic semiconductor

Figure 1 Basic structure for use of the exclusion principle.

Figure 2 Different approaches to high temperature/pressure sensing. Adapted from Ref. 55.

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High pressureSilicon membrane pressure sensors are excellent for low to medium pressure (0300 bar)69, but might easily fail when overloaded due the brittleness of silicon. However, in many industries, such as the automotive and process industries, the pressure to be measured might lie in the range of 20003000 bar. For these higher pressures, alternative membrane material can be used. The main approach uses a metal membrane that determines the pressure range and transfers the value to the sensing element via a uid or xed connection. This method is similar to the approach used in sensors that operate at high temperature. One example deposits polysilicon onto a stainless steel substrate70,

although this method generates additional processing challenges. The best option is to use sputtered oxide as an insulator because it can be formed on a wide range of substrates at relatively low

temperature. The type of stainless steel used was suitable for temperatures up to ~ 500 C. Polysilicon piezoresistors were deposited at low temperature and laser-annealed to avoid excessive heating of the substrate. High pressure/high temperature measurement systems have also been developed using optical techniques71, and ceramics can be applied for this purpose72.

Shock/high accelerationIn some military applications, measurement devices are expected to survive high acceleration and shock during operation. Accelerometers designed for low acceleration still must withstand the high shock/high acceleration, which can be achieved using stoppers that prevent the mass from moving too far. Selected accelerometers use oil in the cavity, which prevents the device from oscillating at resonant frequencies and protects it from shock73.

RadiationMeasurement of radiation or operation in an environment with radiation presents many challenges, depending on the type of radiation. In space applications, devices might be exposed to high levels of ultraviolet (UV) radiation from the sun. Certain systems can be protected from this radiation via installation in the satellite or spacecraft, but others must be exposed to be able to measure. In satellite applications, the devices might also be exposed to sharp swings in temperature. In medical applications, the devices could be exposed to X-ray or proton bombardment. Modern lithography uses extreme UV, which is reected by at least eight Bragg reector mirrors that compose the reticule stage and the wafer stage in the optics of an EUV system. Still, the current mirror reectivity record is still only 70.3% of the EUV light. Current state-of-the-art mirrors are produced by e-beam evaporation and ion beam sputter deposition of superlattices (Mo/Si; see Figure 5

Ti Pt

Sapphire

Ceramabond

Optical fiber

Figure 3 Optical ber pressure sensor. On the basis of Ref. 60.

Figure 4 In situ monitoring of a steam-assisted gravity drainage (SAGD) well with unique white light FabryProt interferometer ber-optic pressure and temperature sensors constructed from highly durable and corrosion-resistant sapphire material. Courtesy of Opsens Solutions Inc., Quebec, Canada.

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Figure 5 Principle and cross-sectional TEM images of a multi-layer UV mirror currently grown by e-beam evaporation and ion beam sputter deposition75,76. Courtesy: F. Bijkerk. TEM, transmission electron microscopy; UV, ultraviolet.

a

b c

Figure 6 Effects of X-ray radiation on MOS transistor characteristics78.

MOS, metaloxidesemiconductor.

(Ref. 74)) with B4C interlayer diffusion barriers to improve thermal diffusion resistance75,76. These devices yield 470% EUV reectivity, although 75% yield is theoretically possible with superior mirror surface protection and new absorber and spacer material layers. In addition to the ultrathin layer deposition techniques that are currently used in EUV mirror manufacturing (for example, e-beam evaporation and ion beam sputter deposition), atomic layer deposition can be developed to further improve the so-called spectral purity of the multilayer stacks. Examples are found in anti-reection coatings designed to improve DUV contrast for inspection (Al2O3, SiON), absorber layers to absorb EUV for image formation (for example, Ta(B)N, TaSi(N)), and buffer/capping layers to protect the mirror stack from absorber-etch damage and from unexpected oxidation, and blistering due to hydrogen radical diffusion into the mirror layer stack, for example, Ru, TiO2, Nb, and

B4C (Ref. 77). Efforts have also been put forth to increase the emissivity (currently ~ 30%) of the layer stack to counteract the heating by IR and DUV radiation from the EUV source.

X-ray radiation can have notably harmful effects on operation of MOS devices, and exposure to X-rays can shift the transistor characteristics78. The shift with increasing exposure is shown in Figure 6 (Ref. 78). This effect can be greatly reduced through layout design using an enclosed layout (Figure 7 (Ref. 78)) with the resulting improvement in radiation hardness shown in Figure 8 (Ref. 79).

Alternative materials can be used to measure radiation signals outside the normal range for silicon. One such approach uses GaN photodiodes formed on top of a standard IC. This approach enables UV measurement, and readout can be accomplished using standard IC technology. A cross-section of this device is shown in Figure 9 (Ref. 79).

Harsh chemical environmentAggressive chemicals affect not only the measurement layer but also the packaging. It is therefore essential that all aspects of the device are considered during development. In certain cases, a layer is used to react with the target chemical to be measured. Many different types of sensors are available for chemical measurement, including electrochemical, chemFET, and optical types. The chemFET option is one of a group of devices that also include devices for measuring ions (ISFET) and enzymes (ENFET). These devices are based on standard MOS transistors in which the gate has been modied to enable the measurement80. Because this device is based on silicon technology, it has a limited

Figure 7 (a) Cross-section of in-pixel elementary devices, (b) regular layout of a MOSFET, (c) enclosed layout of a MOSFET78. MOSFET, metaloxidesemiconductor eld-effect transistor.

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1E3

1E4

1E5

1E6

1E7

1E8

1E9

1E10

1E11

1E12

1E13

1E14

1E15

4 3 2 1 0VGS (Gate voltage) (V)

Vsource =0 V

Enclosed layout transistor (ELT)

I DS(Drain current) (A)

m

1 2 3 4

Figure 8 Reduced radiation effects using the enclosed layout transistor78.

Precision in harsh environments P French et al

Figure 9 Cross-section of a UV sensor that uses a GaN diode. Adapted from Ref. 79. UV, ultraviolet.

Figure 10 Package designed for high-pressure applications in a saline environment83. Reproduced with kind permission from Y Gianchandani and Tao Li.

Figure 11 Capacitive ammonia sensor using porous SiC as the sensing layer and SiC as a protection layer for the electronics.

temperature range. Tobias et al81 reported useful properties at the interface between oxide and SiC at temperatures greater than 700 C (Ref. 81). A review of MOS-based hydrogen sensors using SiC technology can be found in Ref. 82. In certain cases, the aggressiveness of the chemical requires a protection layer or a measurement layer that can withstand the environment. Such a solution is depicted in Figure 10 (Ref. 83).

Silicon carbide is an excellent candidate for harsh chemical environments because it is chemically inert and can also operate at high temperatures. In its porous form, it can be used as an ammonia sensor. Ammonia is highly aggressive and can destroy both many sensing materials, such as silicon, and commonly used metals, such as aluminum. Porous silicon carbide can be used as a capacitive ammonia sensor with high reliability84. The basic

structure is shown in Figure 11. The entire chip is covered with SiC as a protection layer, and only the area above the electrodes is rendered porous for sensing.

HumidityHumidity has always been an issue for ICs. Packaging must be hermitically sealed to ensure that humidity cannot reach the chip. Humidity results in corrosion of the metallization and even creates short circuits if it reaches under the passivation layer to the silicon itself. Humidity sensors must be exposed to this environment and therefore can suffer from reliability problems. Most humidity sensors are capacitive and use polymers or ceramics to absorb the water vapor and therefore change the capacitance8589. With all

of these devices, the remainder of the chip must be hermetically sealed to ensure reliable operation, but a further reliability issue remains with these devices. Both of these materials expand with absorbed moisture, which results in physical expansion.

The continual expansion and contraction leads to stress on the adhesion between the sensing layer and the underlying substrate, which can lead to failure. One solution to this problem is to use porous silicon90. Porous silicon does not expand with moisture absorption, and therefore, the problem is resolved. However, if exposed to high humidity and elevated temperature, surface oxidation in the pores can lead to drift. A move to silicon carbide and use of a structure similar to that in Figure 11 might aid in solving the problem because SiC is far less reactive than silicon and therefore does not oxidize at these temperatures91.

Medical implant/cathetersFor medical implants, limitations exist on both sides. From the medical point of view, there are limitations on the materials that can be used and on the size and shape of the device to maintain biocompatibility, whereas from the device point of view, the implant must survive in a harsh environment. The materials that can be used in medical implants are limited by the constraints of biocompatibility. In many cases, coating layers are applied, such as polymers, parylene, PEEK (polyether ether ketone) or graphene9295. A number of metals can also be used (depending on the location of the device), such as platinum, titanium, and TiN. An overview of the important issues with metals, ceramics, and polymers is given in Table 7 (Ref. 94).

The regulations depend upon the function of the device/ material, where it is located, and for how long it remains in contact with tissue. The longer the contact with tissue, the tighter the regulations will be. Catheters, for example, will usually remain in the body for less than 72 h. Monitoring systems used after an operation might be in contact with tissue for days or perhaps a number of weeks. Long-term implants, such as cochlear implants and pacemakers, are typically implanted for 710 years. The FDA regulations depend on the location of the implant and the length of time that the device is in contact with living tissue. The three categories are o1 day, between 1 and 30 days, and 430 days.

These regulations are covered under the standards ISO 10993-1 in the US96 and EN 30993-1 in Europe.

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Table 7 Brief comparison of metals, ceramics and polymers for implantable applications94

Properties Metal Ceramic Polymer

Biocompatibility Good for limited metals

Good for bio-ceramics

TESTING AND RELIABILITYIf any device is to achieve commercial success, the reliability of the devices must be demonstrated. Where possible, the testing should occur under more extreme conditions than in the intended application. Standard testing methods include high temperature/ high humidity, temperature swings, radiation exposure, high salt solutions, and so on. These tests must be designed to best test the microsystem for the intended environment. At the European Space Agency, a wide range of test facilities is used to ensure that the devices can survive the environment113. In medical implants,

biocompatibility tests are complicated and extensive96. In addi

tion, the devices must be tested to maintain good functionality in the environment. Many tests are performed in salt solutions that are more concentrated than those encountered in applications. Reliability testing is an essential component of the development and must be adapted for each application. This requires a good understanding of the environment and the failure mechanisms114.

CONCLUSIONSA harsh environment is any environment that impedes the normal operation of a sensing or actuating device. A number of approaches can be used to address these problems, such as alternative materials, coating layers or design as well as protective device packaging. For each application, it is necessary to consider all of these aspects to successfully design and fabricate a reliable system.

ACKNOWLEDGEMENTS

We thank our colleagues, past and present, who have contributed to this study and also those who have given permission for the use of gures in this study.

COMPETING INTERESTS

The authors declare no conict of interest.

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Hermiticity Good Medium Mostly poor Degree of outgassing Low Low Mostly high Mechanical exibility Poor Poor Good Reliability Good Good Mostly poor Optical transparency Poor Mostly poor Good (for some) RF transparency Poor Good GoodEase of processing Difcult Difcult GoodCost High High LowRelative weight Heavy Medium Light

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