Report
SILICON MICROMACHINING Sensors to Systems ince the 1950s, silicon has been widely used to fabricate transisW tors and, more recently, integrated circuits containing millions of transistors on a single chip. With the use of parallel photolithographic techniques, which can make hundreds or even thousands of circuit chips simultaneously on a single wafer, tremendous economies of scale were realized over the serial fabrication techniques used previously. As transistor geometries have continued to shrink steadily enormous and previously unimagined densities of electronic circuits continue to be easilv and economically integrated onto sinele silicon chips. It is because of this high level of integration that today's hieh-oerformance computers consist mainlv of nackaging, notwiring, as was the case in their early vears. During this period of rapid growth, engineers realized the tremendous potential of using lithographic, etching, and other fabrication tools developed for integrated circuit manufacturing to make mechanical rather than electronic structures (i). In the late 1950s, for example, mechanical strain gauges were fabricated from silicon wafers and physically glued onto large mechanical components to create a pressure sensor Althouorh these sensors were expensive because of the complex handassembly operations they were quickly adopted for critical aerospace and industrial aDDllcations because of their small size and superior performance 0003-2700/96/0368-407A/$12.00/0 © 1996 American Chemical Society
Although traditionally anengineer'stechnique, micromachining has the potential to enable new analytical applications Gregory T. A. Kovacs Stanford University
Kurt Petersen Cepheid
Michael Albin Perkin Elmer
As the technology progressed, more and more of the mechanical functions of the sensors were incorporated into the silicon chip, including the deformable diaphragm, the diaphragm support frame, wire-bonding pads, and a stress-isolation "constraint" structure that protected the pressure-sensing diaphragm from extraneous packaging stresses. All of these features were built into the chips by using the same parallel-processing, massproduction techniques that have made the integrated circuit so cost-effective. With most of the mechanical and electronic elements of a pressure sensor integrated on a single mass-produced chip, it became possible to use micromechanical sensors in two key high-volume applications (2). In the early 1980s, the automotive industry began manufacturing manifold absolute pressure sensors for the newly mandated emission control systems, which soon became indispensable for automobile pollution control. Today, more than 25 million micromachined manifold absolute pressure sensors are made worldwide each year At the same time, silicon micromachined pressure sensors began replacing reusable blood pressure sensors in the operating room. Earlier models of blood pressure sensors were costly and delicate, and they required expensive sterilization between uses. Because silicon chips could now be manufactured inexpensively and in high volume, the sensor could be
Analytical Chemistry News & Features, July 1, 1996 4 0 7 A
Report thrown away instead of being sterilized and re-used, thereby minimizing the possibility of infection. More than 20 million micromachined disposable blood pressure sensors are sold worldwide each year. These two successful market applications alone have transformed the micromachining industry from a "cottage" technology to a healthy, self-sufficient, expanding industrial sector with numerous emerging applications. For example, today each automobile has one micromachined pressure sensor; by the turn of the century, each automobile may have as many as seven to ten. The number of medical applications such as respirometers infusion pumps catheter-tip pressure sensors and consumer blood pressure measurement is also increasing Even high-volume consumer applications such as wristwatch barometers and handheld
terial from a piece of stock to form the desired geometry and numerous molding procedures to form structures. Subtractive processes include mechanical (cutting, grinding, shaping), electrochemical (electric discharge machining), and optical (laser etching) methods. Comprehensive information on several of these processes can be found elsewhere (4).
tire-prpcmirf* cpnsors now IISP microma-
chined pressure sensors Initially, applications of micromachining techniques were focused on replacing conventional macroscopic sensors with smaller, better performing, and less expensive devices. A more exciting application of micromachining technology, however, is the development of devices for which there are no conventional counterparts. These applications are truly "enabled" by micromachining. One example is the micromirror projection display. This single chip contains an array of up to 4 million tiny moveable mirrors that can be individually tilted on or off at high speed by applying a voltage (Figure 1) When activated in sequence the mirrors can project color video images onto a screen (3) Today, micromachined structures are broadly (and somewhat inaccurately) referred to as microelectromechanical systems (MEMS). Because the majority of the early researchers developing micromachining technology and devices had academic backgrounds in electronic engineering, and later in mechanical engineering, application of micromachining to chemistry and biotechnology has only recently been seriously undertaken. As more scientists with a working knowledge of engineering and for example molecular biology are applying micromachining to their fields exciting results are being obtained at a rapid pace This growth has 408 A
Figure 1 . Texas Instruments digital micromirror device. (a) One mirror element in (b) an array of mirror elements.
A micromachining primer One of the major strengths of micromachining is the reproducibility that can be achieved in manufacturing. Typically, each silicon wafer will ultimately yield hundreds or even thousands of individual "chips." The basic concept is to sequentially superimpose two-dimensional patterns onto wafers, with each pattern representing a step of deposition or etching of deposited layers or the substrate itself. The final structures often three-dimensional result from the combination of these additive or subtractive steps (2) These processes are extremely reproducible at the wafer-scale (Note that in micromachining literature structures formprl from thin films above the mirface of ttie cnrictrate are refprrArl to as " s u r
been fueled by thefindingthat nonsiliconbased micromachined structures can be used alone or in conjunction with silicon. We believe that such interdisciplinary work will lead to so many new micromachined devices and systems that they may eclipse the present dominant market position of micromachined pressure and acceleration sensors. There is a great deal of current work in "wet" micromachined devices, including valves, micropumps, flow channels and restrictors, cellular probes and manipulators, bioreaction chambers, and chemical separation columns. In this Report we present an overview of basic micromachining processes briefly compare them with other familiar machining tools, and discuss commercial MEMS applications. Another article in the near future will address micromachining applications in the biological and biomedical fields. Researchers should bear in mind that there are several standardized mechanical fabrication methods available, some of which can be used to achieve small dimensional features. These include subtractive processes that selectively remove ma-
Analytical Chemistry News & Features, July 1, 1996
face" micromachined, and structures formed bv ptching awav significant regions of the supporting substrates arp referred to as "bulk" micromachined.) Photolithographic patterning is nearly always used to define regions where additive or subtractive processes act (Figure 2). Typically, photosensitive polymers, collectively referred to as photoresists, are spin cast onto the wafers and baked to drive off solvents. The photoresist layer is then exposed to UV light through a mask (analogous to a photographic master or negative) and is selectively removed after developing. The pattern of the photoresist is then used to define one layer of the structure being fabricated by exposing regions (on previously deposited layers or on the substrate itself) to be removed by chemical etching. Once that step is completed, the photoresist is removed. Additional layers can be deposited, followed by another photoresist application and patterning, which is followed by another subtractive etching step. This basic series of three steps (deposition, photoresist patterning, and etching) is repeated until the entire fabri-
into the silicon crystal lattice by diffusion or ion implantation is extremely important for formation of active electronic circuit elements and stress-sensitive resistors and for defining regions where certain aqueous etchants do not act. Bonding, or lamination of sheets of maAdditive processes A wide variety of additive processes can be terials or additional wafers above the substrate, is also used for additive processused for micromachining (Table 1). The ing. Anodic bonding uses electrostatic atsimplest process is more properly a reactive process: the thermal growth of silicon traction to bring a glass wafer into contact with a silicon wafer so that covalent dioxide on the surface of the substrate bonds can form between the glass and the (5). Because thermal oxidation of silicon occurs at temperatures high enough to de- silicon (10). Silicon fusion bonding makes use of a similar bond-formation mechastroy subsequent structures (> 900 °C)) nism between the slightly oxidized surthis step is typically done first if needed. faces of two silicon wafers at high temperFor subsequent deposition of electriature (11). Both of these techniques are cally insulating dielectrics such as silicon used extensively in the manufacture of oxides and nitrides, lower temperature pressure and acceleration sensors processes can be used. In general, reactant gases are introduced into a chamber containing the wafers, and either thermal energy (as used in chemical vapor deposition, CVD, 2) or electron bombardment (as used in plasma-enhanced chemical vapor deposition, PECVD, 5) is used to drive the reaction to form the desired compounds. Such processes can be used to deposit not only silicon-based dielectrics but also polycrystalline or amorphous silicon as well as organic and metallic films cation sequence is completed. For some integrated circuits, the sequence may be repeated 25 times or more, each time with a different deposited material and a different mask pattern.
In addition, the surfaces of micromachined structures can be modified through surface chemistry to change properties such as wettability, adhesion, surface adsorption, and surface reactivity. Techniques involving the addition of selfassembled monolayers (12) or the attachment of complex macromolecules such as immunoglobulins and nucleic acids (13) are creating new opportunities for MEMS in a variety of application areas. Subtractive processes
The simplest subtractive micromachining processes use aqueous etchants and are differentiated on the basis of whether they are isotropic or anisotropic (Table 2). Many wet etchants exist for silicon, including isotropic HNA (HF:HN03:H20) (1) and anisotropic hydroxides of alkali metals such
Plasma-driven deposition is often used when low-temperature (100-300 °C) processing is desired. The driving frequency of the plasma can be manipulated to control film stoichiometry and stress. Such low-temperature processing is essential whenfilmsare to be deposited on top of prefabricated circuitry with aluminum interconnections. Because aluminum is typically used to interconnect the electrical components in circuits, subsequent processing steps must occur at temperatures low enough (< 450 °C) not to melt or otherwise damage it Other metals (e g titanium nickel and tungsten) may be used to alleviate these constraints Other options for additive processing include electroless plating (6) and electroplating (7) of metals, spin casting of dielectrics and polymers (5), and localized laser-driven deposition (8). For biotechnology applications, spincasting and silk screening can be used to deposit organic matrices containing bioactive compounds such as ionophores to form biosensors (9). Finally, the insertion of dopant atoms
Figure 2. Basic micromachining processes. (a) Exposure of photoresist to UV light, (b) After exposure with photoresist removed, (c) Lift-off deposition, (d) Anisotropic dry etch, (e) Anisotropic wet etch. Analytical Chemistry News & Features, July 1, 1996 4 0 9 A
Report
as KOH (14), tetramethyl ammonium hydroxide (TMAH, 15), and ethylenediamine and pyrochatechol in H20 (EDP, 16). These etchants all dissolve silicon by oxidation to silicates. The anisotropic etchants show a marked dependence of reaction rate on silicon crystal planes, resulting in the exposure of slower etching planes (and hence anisotropy, or "orientation dependence"). Although impressive degrees of anisotropy can be achieved through these crystal-plane-dependent mechanisms, the designer is limited by the fixed crystallographic angles in the single crystal silicon. The anisotropic etchants also show large decreases in etch rates in regions of the silicon that have been dooed with hitrh concentrations of o-tvoe dooants such as boron (17) Both isotropic and anisotropic silicon wet etchants can be sensitive to applied voltages. For example, when an appropriate electrical potential is applied to an "n-doped" silicon region, the silicon surface in that region will form an oxide in the etch solution, and this anodically grown oxide will inhibit further etching (18). Such electrochemical etching processes can be used to completely remove a p-type substrate while preserving a thin n-type surface-doped layer. This technique is routinely used to create the thin singlecrystal silicon mechanical beams and diaphragms used in commercial acceleration and pressure sensors Such etch-
stop regions can be very useful in the fabrication of many other types of micromachined structures (19). Isotropic and anisotropic "dry" or plasma-based etch processes (referred to as plasma etching or reactive ion etching, depending on the etch mechanism) also exist for silicon, dielectrics, organics, and metals. In the case of plasma-based etching, the relative anisotropy of a given process is determined by the plasma chemical mechanisms needed to protect the sidewalk of an etched hole (to prevent lateral etching) and by the dependence of etching on bombardment by energetic ions arriving perpendicular to the wafer's surface. Extremely high degrees of anisotroov (depth/width ratio for an etched groove) can be achieved by using plasma-based etches (5) Laserenhanced chemical etching can be used to etch nearly arbitrary geometries but this process is relatively slow compared with
conventional methods (20) Combining additive and subtractive processes Additive and/or subtractive processes can be combined to create "compound" processes. For example, making sacrificial layers (underlying deposited layers that are later etched away to form gaps) falls into this category. The only requirement is that the sacrificial layer must be etched away without destroying any of the desired structures made from other materi-
Table 1. Additive processes.
Thermal (9001100°C)
CVD (6001000 °C)
PECVD (100600 °C)
Sputter (0300 °C)
X
X
X
X
Wafer bonding (Si-glass, Si-Si)
Spincast or screen printing (RT)
Electroplating (RT)
Insulators
Silicon dioxide nitride conductors
Single
X
1000 °C
nr\/Qtal
silicon Metals (Al, Au, Cr) Polysilicon Amorphous silicon Aolymers
410 A
X
X
X
X X
X
X
Analytical Chemistry News & Features, July 1, 1996
X
als. Examples of sacrificial layer processes include wet etching of silicon dioxide using HF to release polysilicon structures (21,22) or to form cavities (23) and oxygen plasma etching of sacrificial organic layers to release aluminum structures (3). Another approach to cavity or gap formation is bonding or laminating other wafers above prefabricated depressions in the bottom wafer. Wet or dry etching can be used to create the depressions in the bottom wafer, then anodic or silicon fusion bonding can be used to seal the wafers (24). After the bonding step, further etching can be used to selectively open the sealed cavities to form movable structures with anchors to the substrate in desired locations (25). Template-based processes involve selectively etching away regions of a "template layer" and filling the regions with a different material. For example, metals can be electroplated in such voids after etching patterns in an organic template layer such as photoresist or polyimide (26). A high-aspect-ratio template-based process called LIGA (lithographic galvanoformung, and abformung) uses synchrotron X-ray radiation to expose thick cast layers of poly(methyl methacrylate) on the substrate; metals such as nickel or Permalloy are then electroplated into the voids formed when the exposed poly(methyl methacrylate) is removed (27) There are many other specialty micromachining processes that will not be discussed in depth here. Some of the more important ones are micromolding (to form structures in alternate, moldable materials) using LIGA or other etched structures as masters (28), out-of-theplane unfolding of surface-micromachined structures to form three-dimensional devices (29), and shadow-masking to form ultrasharp tips such as those used for field emiision displays s30). The diversity of structures and materials that can be integrated by using micromachining methods is illustrated in Figure 3 Some of the size, density, and integration advantages provided by silicon-based technologies are not always obvious when visually examining a tiny part. A key step in moving forward is to place these small parts in environments where they can be used practically and, in some cases, connected to each other.
the application of pressure or acceleration forces, in addition to electrical testing. For testing, most transducers must Wet orientation be exposed to the form of energy they are Wet isotropic etching dependent etching Plasma etching designed to sense. For example, optical Dopant ElectroDopant ElectroIsoAnisosensors need to be tested with light (easy Normal selective cn©miC3l Normsi ssiective cnsmical tropic tropic on a wafer scale), and accelerometers Silicon HF, HNA HNA KOH, KOH, KOH, X X Nitric, TMAH, TMAH, TMAH need to be accelerated. In the case of some acetic EDP EDP accelerometers, such as the airbag accelctCIQS erometer shown in Figure 4 (top), electriSilicon i~ir* A X cal test signals can be used to mechanically deflect the accelerometer element to simulate real acceleration. Such substiAl s Ph h ' X X tute testing methods are not generally posacetic, sible for transducers particularly for chemical A self-test strategy that acids _. H „ . has been successfully anolied to micror IH KI I machined chemical transducers such as P i \/, 2 0 the clinical chemistry analwer shown in Figure 4 (bottom) is to automatically puncture a sealed packet of calibration retime test and caliPackaging issues many times greater than those of manufacbrate the sensors prior to exoosure to the A crucial aspect of micromachined compo- turing the devices themselves. This fact solution to be tested (97) is often overlooked when the "low cost" of nents that must be considered at the outset of any application is packaging. The nu- batch-fabricated micromachined devices As do other electromechanical devices, is quoted. The economies of scale for tradi- many MEMS devices dissipate considermerous advantages of micromachined components can be degraded or might not tional analog and digital integrated cirable power. In many cases, the power recuits are tremendous because low-cost, even be attainable without proper, costquired for sensing, actuation, or other uses effective packaging. This problem is intrin- commercial-grade packaging is available, is not directly related to size, and it may preshipment testing is carried out on the be necessary for many microstructures to sic to micromachining technology simply because the parts are often very small and entire wafer, and the testing is purely dissipate the same power in a smaller electrical. Micromachined devices howrequire comparably minute ports to the volume, thereby giving rise to increased ever, usually require other testing, such as power densities and potentially detrimenoutside world for access to gases, fluids, and so forth These packages are not currently produced in volumes anywhere those of packages for conventional integrated circuits (and are therefore potentially expensive) Table 2. Subtractive processes.
Microsensors are particularly sensitive to extraneous stresses that can be induced during the process of packaging. In addition, ambient temperature variations induce further stresses because of mismatches in the thermal expansion coefficients of the various packaging and microcomponent materials. Stress control by mechanical design of the package and by choice of materials are crucial to the practical use of micromachined devices. Particular attention must also be given to small particles that TT13V block or interfere with a micromachined feature sitating very clean environments for fabrication and assembly in addition to desion considerations
The costs of packaging and testing micromachined transducers are generally
Figure 3. Combination of micromachining processes in a single hypothetical device. Two 0.5-mm silicon parts held together by a fusion bond or thin oxide layer and containing (a) anodically bonded glass, (b) surface-micromachined polysilicon, (c) CMOS circuitry, (d) electroplated metal, (e) a deep reactive ion etch, (f) an anisotropic wet etch, and (g) an isotropic wet etch. Analytical Chemistry News & Features, July 1, 1996 411 A
Report (11) Barth, P. W. Sens. Actuators 1990, A21A23,919. (12) Kumar, A; Abbott, N.L.; Kim, E.. Biebuyck, H. A; Whitesides, G. M. Acee Chem. Res. 1195,28,219. (13) Beaucage, S. L; Iyer, R P. Tetrahedron 1992,48,2223. (14) Bean, K E. IEEE Trans. Electron Devices 1978, ED-25,1185. (15) Tabata, 0.; Asahi, R; Funabashi, H.; Sugiyama, S. Presented at Transducers '91, International Conference on Solid-State Sensors and Actuators Digest of Technical Papers; IEEE Press: San Francisco, Future opportunities 1991, pp. 811-14. Naturally, a key to the commercial success (16) Finne, R. M.. Klein, D. L.J. Eledrochem. Soc. 1967,114,965. of a micromachined-based system is whether the individual components can be (17) Bohg, A/. Elecrochem. .oc. 1971,118, 401. built and assembled cost-effectively for (18) Kloeck, B.; Coffins, S.; de Rooij, N.; Smith, the application. Along with automotive apR. L IEEE Trans. Electron Devicev 1989, 36,663. plications, research on MEMS for aero(19) Reay, R. J.. Klaassen, E. H.; Kovacs, G.TA space, communications, and data storage IEEE Electron Device cetters 1994,15, is underway. We expect that biotechnol399. (20) Bloomstein, T. M.; Ehrlich, D. J. Appl. ogy applications of micromachining will Phys. Lett. 1192, 61, 708. be a tremendous area of growth in the (21) Mehregany, M.; Gabriel, K. J.; Trimmer, coming decades. As micromachined deW.S.N. IEEE Trans, on Electron Devices 1988,35, 719. vices chemistries packaging and other system-building tools improve more fully (22) Fan, L-S.; Tai, Y-C; Muller, R. IEEE Trans. Electron Devices 1988,35, 724. integrated chemical analysis and invasive (23) Guckel, H.; Burns, D. W. Proc. IEEE International Electron Devices Meeting 1984, medical systems will be realized Micro223-25. machining has the potential to enable (24) Petersen, K; Barth, P.; Paydock, J.; new annlications provide portable lowBrown, J.; Mallon, J.; Bryzek, J. Technical power systems and new low-cost Digest, IEEE Solid-State Sensor and Actuator Workshop 1988,144. disposable analytical annlica(25) Kassen, E. H., et al. Presented at Transducers '95, International Conference on Solid-State Sensors and Actuators; Stockholm, Sweden, 1995; pp. 556-63. We would like to thank Dick Thomas for pre(26) Frazier, A B.; Allen, M. G.J. Microelectroparing the illustrations, Reid Kowallis for technimech. Syst. 1193,3,87. cal input, and Charles Sloan and Nadim Maluf (27) Ehrfeld, W.; Gotze, F.; Munchmeyer, D.; for reviewing and critiquing this paper. Shelb, W.; Schmidt, D. Technical Digest, IEEE Solid-State Sensor and Actuator Workshop p988,1-4. (28) Becker, E. W.; Ehrfeld, W.. Hagmann, W.; References Maner, A; Munchmeyer, D. Microelec(1) Petersen, K. E. IEEEProc. 1982, 70,420. tron. Eng. .986,4, 35. (2) Bryzek, J.; Petersen, K.; McCulley, W. (29) Pister, K.S.J., et al. Sens, and Actuators IEEE Spectrum 1194,20--1. 1992, 433,249. (3) Hornbeck, L J. Proc. .PIE Int. Soc. Opt. (30) Spindt, C. A, et al./. Vac. Sci. Technol. Eng. 1990,1150,86. 1993,11,468. (4) Doyle, L E. Manufacturing Processes and (31) Erickson, K A; Wilding, P. Clin. Chem. Materials for Engineers; Prentice Hall, 1993,39,283. Inc.: Englewood Cliffs, NJ, 1985. (5) Runyan, W. R.. Bean K. E. Semiconductor Integrattd Circuit Processing Technology,Gregory T. A. Kovacs is aa assistant profesAddison-Wesley: Reading, MA, 1990. (6) Electroless Plating: Fundamentals and Ap-sor of electrical engineering at Stanford University. Kurt Petersen is sofounder and plications; Mallory, G. 0.; Hajdu, ,, B., president ofCepheid, a company that develEds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. ops new applications for micromachining (7) Harrison, J. A; Thompson, J. Electrochim. technology. Michael Albin ii a senior rtaff Acta 1973,18,829. scientist in the science and technology group (8) Westberg, H.; Boman, M.; Johansson, S.; Schweitz, J. A/. Appl. Phys. 1193, 73, at the Applied Biosystems Division ofPer7864. kin Elmer. Address correspondence about (9) Goldberg, H. D.; Brown, R B.; Iiu, D. P.; this article to Albin at Applied BiosysMeyerhoff, M. E. Sens. Actuators B1994, tems, 850 Lincoln Centre Dr., Foster City 21,171. (10) Wallis, G.; Pomerantz, D. I.J. Appl. Phys. CA 94404 (E-mail albinmn@perkin-elmer. com). 1969,40,3946. the micro-to-macro interface) be eliminated? If a microfluidic chip must be connected to a microsensor chip, what is the best method to merge the components without compromising their performance? If these issues are not addressed correctly, the potential advantages of the micromachined components may not be realized.
Figure 4. Examples of packaging. Accelerometer (courtesy of Analog Devices) and clinical analyzer cartridge (courtesy of i-STAT Corp.).
tal thermal effects. These issues are of special concern in chemical analysis, biomedical analysis, and biological handling microsystems where undesirable heat can denature or otherwise damage compounds of interest. At some point in any application of micromachined devices there must be a micro-to-macro interface, and the designer must consider several key questions. How should a bottle of reagent be connected to a microvalve or a fluid restrictor chip with a 25-um internal radius without clogging? Can reagent volumes be scaled with the micromachined instrument components? (If not, an instrument with an ultraminiaturized analytical core may be the same size as its conventional equivalent.) How should filters be attached to or incorporated into such microdevices? Can potential dead volumes or crevices (at 412 A
Analytical Chemistry News & Features, July 1, 1996
