SCREAM MicroElectroMechanical Systems

ELSEVIER

Microelectronic Engineering 32 (1996) 49-73

SCREAM MicroElectroMechanical Systems Noel C. MacDonald School of Electrical Engineering and the Cornell Nanojabrication Facility. Cornell University. Ithaca, NY 14853, USA

Abstract A process called SCREAM fur ,2ingle ~rystal Reactive £;tching ~nd Metallization is used to make MicroElectroMechanical Systems (MEMS). The SCREAM process yields high-aspect-ratio (>50: I) released, single crystal silicon structures with micrometer-scale minimum features and a suspension span of greater than 5 millimeters. Such structures include picofarad sensing capacitors and high force actuators that generate milli-Newton forces (100 mN I em' at 40 Volts) and controllable. three dimensional motion and displacements. Examples of SCREAM devices and microinstruments include a materials testing or loading instrument, and micro-scanning tunneling microscopes.

Keywords: SCREAM; MicroElectroMechanical Systems (MEMS)

1. Introduction

The field of MicroElectroMechanical Systems (MEMS) is a very broad field which includes micrornachining fixed or moving microstructures; and includes micro-electro-mechanical, microfluidic, micro-optical-electro-mechanical and micro-thermal-mechanical devices and systems. MEMS usually consist of released microstructures that are suspended and anchored, or captured by a hub-cap structure and set into motion by mechanical, electrical, thermal, acoustical, or photonic energy source(s). The integration of MEMS and microelectronics is a significant paradigm. Moving microstructures and MEMS have been made using etched single crystal semiconductors, deposited thin films, and plated metals. Two popular micromachining methods [I] are surface micromachining [2] and bulk micromachining. A popular surface micromachining technique uses a thin film (2 urn) polysilicon layer [1,21 supported by a silicon dioxide sacrificial layer on a substrate which is usually silicon. After patterning the polysilicon layer to delineate the suspended or moving portions of the structures, the sacrificial layer is removed using a wet chemical etch; for example, a buffered solution of hydrofluoric acid is used to remove the silicon dioxide. The movable polysilicon microstructures are suspended and anchored to the substrate; rotating polysilicon microstructures [3] such as micromotors and microgears are captured in a hub-cap structure and are free to rotate around the hub. The polysilicon-based surface micromachining process has received widespread acceptance for making small (lOOs of micrometers on-a-side) microstructures, low force « 100 I1N) actuators and sensors. A three-layer polysilicon process has been implemented. A second, popular approach to micromachining is bulk micromachining of single crystal silicon [1,4,5] or quartz [1,6]. Bulk 0167-9317/96/$15.00 Copyright © 1996 Elsevier Science BY. All rights reserved SSDI: 0 167-9317( 96 )00007-X

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micro machining is used to make larger MEMS and to make stationary three-dimensional structures. The bulkmicromachining processes include wet chemical etching, reactive ion etching (RIE) or both to form released or stationary microstructures. Bulk micromachined structures made using wet chemical etching depend on the directional etch properties [4,5] of the wet chemistry on single crystal materials and produce structures and shapes defined by the convolution of the mask pattern and the etch properties of the crystallographic planes. Thus, closely spaced structures with micrometer-scale dimensions are difficult to fabricate. Sharp corners defined by the crystallographic plane can contribute to structural failures. For some large mass or large area structures such as fluid channels and micro-valves, wet chemical micromachining together with wafer bonding have been used to fabricate a number of devices and MEMS [7]. The dissolved wafer process [81 is a bulk micromachining process that uses boron-doped epi layers and boron diffusions to create wet chemical etch stops to delineate released structures; microstructures are also formed using RIE. Another MEMS process called LIGA [9,101 uses a plating process to fabricate high-aspect ratio metal and polymer parts or molds. All these processes are reviewed in two recent texts 11, II]. The reader is also directed to a number of journals [12-15] and conference proceedings [16-181 that record the diversity of MEMS structures and processes and provide a window to new theoretical and applied results. For an excellent review of MEMS activities prior to 1982, the classic paper by Kurt Peterson , "Silicon as a mechanical material" [19] is highly recommended. Reviews of gallium arsenide as a mechanical material [20] and microfl uidics [21] provide a window to these areas of MEMS. Possible paradigms and directions for future MEMS are discussed in a recent paper [22] by K. Gabriel: " Engineering microscopic machines" . The subject of this paper is a bulk micromachining process called SCREAM for ~ingle ~rystal Reactive gtching And Metallization process. The process has been used to fabricate released microstructures from both single crystal silicon (SCS) [23,24J and single crystal GaAs [25]. This paper focuses on SCS SCREAM processes and devices. SCREAM processes and process sequences are described, and devices and microinstruments fabricated using the SCREAM processes are highlighted. Emphasis is placed on making high-aspect-ratio microstructures which are used to construct microinstrurnents with high force actuators and large sensing capacitance, large-area micro-optical structures, and MEMS that incorporate three-dimensional (X, Y, Z) displacements and actuation.

2. SCREAM high-aspect-ratio, capacitor-microactuator The energy stored in an electric field of a suspended capacitor is often used to generate force in MEMS . A popular capacitor actuator is a comb-drive rnicroactuator first demonstrated by Tang and Howe [26] in a 2 u.m thin film polysilicon/silicon dioxide sacrificial-layer process. A high-aspectratio, SCREAM version of a comb-drive microactuator supported by lateral , SCS spring is shown in Fig . lea) - A; the large area, SCS (beams) sidewalls are the plates of the capacitors, and the plate-spacing is 1.5 urn, Fig. lea) also shows a suspended parallel -plate capacitor with a movable plate (D) and the fixed plate (E). The capacitors are supported by springs (B) with fixed supports (C). For capacitor sensors and capacitor actuators, the capacitance is proportional to N(b/ d ), where b is the height (Fig. l(b)), a is the width of the plates (beams), d is the plate spacing, and N is the number of plate-pairs. Consequently, high force actuators require a large number of closely spaced high-

N.C. Maclronald / Microelectronic Engineering 32 (/996) 49-73

51

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Fig. 1. (a) A high-aspect-ratio (b/a) single crystal silicon device made using SCREAM processes. [A] - interdigitatedelectrode microactuator that moves the structure to the left when a voltage is applied to the two sets of electrodes. [8] - suspended spring with supports - [C]; [D]- moving suspended plate of a parallel-plate capacitor attached to the spring and a fixed plate - [E]. The parallel-plate capacitator is used to sense the displacement. (b) Schematic of a suspended SCREAM structure where b is the height of the electrode-structure and a is the width of the structure. The Si0 2 insulators on the moving beam, electrically isolate the metal electrodes - the capacitor plates - from the silicon substrate and the silicon core of the beam.

aspect-ratio plates (beams). The maximum displacement of the comb drive actuator is proportional to the length, l, of the comb drive fingers. The maximum value of l (and hence the displacement) is the distance or span that N released and suspended comb-fingers can remain flat and dynamically stable when the voltage is applied to the actuator. High force actuators are made using high-aspect-ratio, very stiff microstructures and the parallel operation of N actuators supported on a stiff backbone structure (Fig. 2). The planarity of the entire released structure during operation is crucial to the stable operation of high force actuators (refer to

Fig. 2. An array of comb-drive or interdigitated-electrode capacitive actuators made using the SCREAM process. The banks of "finger-like-structures" are the comb-drives mieroactuators or motots, and the lattice network is the backbone or support structure for the microactuators. The cantilever beam on the bottom right is one of the springs that support the structure. The entire device has 9000 comb-drives.

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N.C. MacDonald I Microelectronic Engineering 32 (1996) 49-73

Section 4.2.1). Thus, it is very important to make microstructures with large vertical or out-of-plane stiffness. The vertical stiffness scales as (b / l)3 where b is the height of the silicon beam and I is the length or span of the beam. The fabrication of a high force actuator array requires a planar, released structure that can span mm-scale to em-scale dimensions. In general, both small and large structures need to be integrated into the same MEMS. Therefore, the concept of maintaining the minimum feature size of the beams and the minimum microstructure spacing over large mm-scale spans is the key to generating large forces and large sensing capacitances while maintaining structural stability and separation of mechanical modes. The SCREAM process yields mm-scale SCS structures with 1.5 urn minimum feature size beams and beam separations, and with beam heights greater than 100/-Lm yielding an aspect-ratio of bla = 66. Forces in excess of 100 milliNewtons per cm' of silicon at 40 Volts are possible using SCREAM processes and capacitive actuators. These features of the SCREAM process, SCREAM microstructures and SCREAM microinstruments are highlighted in the text that follows.

3. SCREAM processes for single crystal silicon 3.1. Foreword

The SCREAM processes grew out of efforts to make high frequency (MHz) actuators to move SCS microstructures and nm-scale tips in three dimensions. The progenitor or precursor process to SCREAM is a process used to make isolated islands of submicron-silicon by selective lateral oxidation (ISLa) [27]. The ISLa process Fig. 3 uses reactive ion etching of SCS to form submicron SCS structures which are then isolated from the silicon substrate by a lateral thermal oxidation of the bottom portion of the silicon microstructure. Electron beam lithography (Fig. 3(a)) is used to pattern the 200 nm to 500 nm features then the silicon is etched. The SCS beams or microstructures are nitride-coated to prevent total oxidation of the silicon beam (Figs. 3(a) and (bj), The RIE and thermal oxidation process steps (Figs. 3(c) and (d») produce substrate-silicon microstructures on substratesilicon of a nominal width of 300 nm (Fig. 3(e)). The composite beam-structure consists of a nitride-coated SCS beam supported by (~1 urn) section of thermal silicon dioxide attached to the SCS substrate. Fig. 3(f) shows a cross section of an actual SCS structure at step d - Fig. 3(d). During the development of this process, we observed that nm-scale, silicon wedges or silicon tips form before the complete oxidation of the SCS beam as shown in Fig. 3(b). This observation, together with the removal of the silicon dioxide from the bottom of the SCS beams using a hydrofluoric acid release-etch step, generated a new process to fabricate and integrate nm-scale (20 nm diameter) tips on fixed [28] and moving structures [29] - a very important technology for making scanned-probe MEMS. The MEMS process based on ISLa is called COMBAT (~antilevers by Qxidation for ~chanical Iieams And Tips) and has been used to produce a tip-above-a-tip tunneling structure and many other MEMS devices [29,30,31]. The SCS microstructures are released from the substrate using a buffered hydrofluoric acid etch. Fig. 4 outlines the COMBAT process steps [29] used to produce released electrically conducting and electrically isolated microstructures. The lines labeled (d) and (e) show the incorporation of a partial, SF 6 RlE etch of the bottom of the silicon microstructures to reduce the thickness of the beam and thereby reduce the time to oxidize through the bottom of the SCS beam.

n.c. MacDonald I

Microelectronic Engineering 32 (1996) 49-73

53

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COMBAT processes include integrated steps to create (the columns: cross sections A-E) a released beam with an integrated tip column (A); a released cantilevered beam (B); an isolated, fixed silicon beam (C); an isolated, fixed silicon beam - an anchor or support - with a metal contact (D); and an isolated, fixed silicon dioxide beam (an anchor with metal) with metal for electrical connections and contacts along the released silicon beams (E). Fig. 4(b) shows a suspended and moving structure

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N.C. Maclronald I Microelectronic Engineering 32 (1996) 49-73

Fig. 3(£) (Continued)

[29-30] made using the COMBAT process with eight sets of SCS, parallel-plate capacitors suspended by springs with isolation and contacts; electron beam lithography was use to pattern the 300 nm features. Also, a field effect transistor with a 100 nm effective channel length was fabricated on a 500 nm SCS beam using the COMBAT process [31]. The ISLa process (Arney) and the COMBAT process (Yao and Arney) used to form released, movable MEMS provided a technology base that gave birth to SCREAM (Zhang). It was noted that the time to oxidize wider, micrometer-scale SCS microstructures is long; the oxidation front is not uniform for different shaped structures; and the wet etch release process is not a robust release method for larger, more complex MEMS structures. Zhang eliminated the oxidation step and the hydrofluoric acid step for structure-release and replaced these steps with an isotropic SF 6 RIE etch-through of the silicon beam to fully release up to 2 J.Lm wide, optically-patterned beams. SCREAM-compatible metallization, contacts, and isolation steps were added to create the first version of SCREAM [32]. COMBAT exhibits some advantages for making nm-scale structures and can be integrated with larger SCREAM structures. 3.2. SCREAM process

The SCREAM process produces suspended, high-aspect-ratio single crystal silicon microstructures that span distances of many millimeters. Like the COMBAT process, the shapes are independent of crystallographic directions, so even circular SCS structures (Fig. 5) can be fabricated. Fig. 6 illustrates the key SCREAM process steps [24,32] which include an anisotropic etch-step to define the SCS microstructures (Fig. 6, columns A-C, row (a»), and a selective an isotropic release etch-step (Fig. 6, column A, row (i) that separates the microstructures from the SCS substrate to produce both suspended structures and the required supports. Furthermore, the SCREAM process incorporates processes to electrically isolate micro-structures (Fig. 6, columns B and C) for electrical actuation and sensing; an isolated anchor or support is shown in Fig. 6, column C, row (i), Unlike COMBAT where thermally grown silicon dioxide is the release material, the SCREAM process incorporates thermal

NiC. MacDonald / Microelectronic Engineering 32 (1996) 49-73

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Fig. 4. (a) Process flow for COMBAT process including integrated tip-pairs, electrical isolation, electrical contacts, and electrical conductors. Each column (A-E) is the process sequence to obtain the final structure in line (i), listed under column headings (A-E). (b) SEM micrograph of a COMBAT sensor with eight pairs of parallel plate capacitive microactuators. The center "cross" has a "tip-above-tip" structure. The device uses all the process sequences outlined in (a). The 300 nm single crystal silicon beams were patterned using electron beam lithography (refer to Refs. [29-31]).

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N.C. MacDonald / Microelectronic Engineering 32 (1996) 49-73

Fig. 5. Micrograph of Circula r s ingle crystal silicon structures. Patterns made with the SCREAM processes are independent of crystallographic planes . Suspend ed circular structures with springs illustrate " pattern insensitivity" of the SCREAM processes.

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I Microelectronic Engineering 32 (1996) 49-73

57

silicon dioxide isolation segments along the silicon beam (Fig. 6, column B) which are not removed during the SF 6 release step. The integration of electrical (and thermal) isolation along the beam and at the beam-supports, together with suspended interconnections and suspended active electronic devices arc the key additional components of the SCREAM process. The original SCREAM process flow [23] is shown in Fig. 7. The first step is the deposition or growth of a thick masking oxide on the SCS substrate. The silicon dioxide is patterned using standard optical lithography (Fig. 7(a)). Pattern transfer to the silicon is accomplished by an anisotropic chlorine etch to produce silicon structures and trenches. The SCS structures are covered by a thermal or chemical vapor deposited silicon oxide layer to mask or protect the patterned structures during the subsequent release step. Aluminum is sputtered onto the structures for use as contacts and interconnects (Fig. 7(e)). A second mask step (Fig. 7([)) before release requires non conventional trench patterning to remove the silicon dioxide and aluminum from the suspended beam and the bottom of the trench. Finally, an isotropic SF 6 release etch is used to under cut and separate the SCS

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MacDonald / Microelectronic Engineering 32 (1996) 49-73

structures from the substrate (Fig. 7(h)). The released SCS microstructures are detached from and suspended over the substrate and are anchored to and supported by the substrate at the ends of the supporting beams or springs. A key feature of the SF 6 etch-release step is the undercutting of both the suspended silicon beam and the silicon dioxide coated sidewalls of the silicon trench. The undercut of the silicon produces an overhang of silicon dioxide which electrically isolates the deposited metal (aluminum) conductor from the silicon substrate. The SCREAM process shown in Fig. 7 has been further refined to include electrical isolation along the released beam (Fig. 6, column B) and electrical isolation at the beam-support structures (Fig. 6, column C). The electrical isolation is accomplished by a local thermal oxidation of the beam-segments and the beam-supports: (Fig. 6, column B and C). 3.3. SCREAM-I process Another version of the SCREAM process requires only a single masking and photolithography step to define the structure. This version of SCREAM, SCREAM-I [24] (Shaw and Zhang), has advantages for fabricating simple MEM structures: (I) the second mask-alignment step is eliminated; (2) the metal is deposited in the last process step and does not require patterning; and (3) the process is a low temperature process «400°C). Consequently, SCREAM-I MEMS can also be implemented on fully processed VLSI wafers [33]. Fig. 8 illustrates the SCREAM-I process steps. Note, no second mask is required to electrically isolate the released silicon structures (steps 8 and 9), since electrical isolation of the metal (e.g. to form a capacitor) is accomplished with the CVD insulating layer on the silicon substrate and the released beams. Also, the metal deposition (step 10) is the last step in the process and does not require a mask level. Like SCREAM, the undercut of the silicon during the release step produces an overhang (step 10) of silicon dioxide which electrically isolates the deposited metal (aluminum) conductor from the silicon substrate and from the silicon microstructures. Fig. 9 shows a capacitor accelerometer [24] fabricated using the SCREAM-I process. The "white structures" shown in the SEM micrograph are the suspended capacitor plates supported by spring-supports at each end. The detail structure of the capacitor plates is shown in Fig. 9(b). Electrical contacts and bonding to the chip are accomplished by forming mesas for the bonding pads as shown by the wire bond in Fig. 9(a). The detail of a bonding pad or spring anchor is shown in Fig. 9(c). The fixed mesas have the same structure as the movable beam, and the metal bonding pad is isolated from the silicon mesa by the CVD oxide layer and the oxide-overhang or "skirt" formed by the SF 6 undercut etch. Note the bonding pad adds a fixed capacitance which is detrimental for some applications. However, if the device is integrated on a standard silicon chip with electronics, the bonding pads are not required and the parasitic capacitance is much reduced. Process mixes of SCREAM, SCREAM-I, and COMBAT are also possible. 3.4. Scream etch masks High-aspect ratio SCREAM structures are made using a patterned thick oxide mask and a deep anisotropic etch (refer to Fig. 7, steps (a)-(c); and Fig. 8, steps 1-5). For very deep etching (>20 urn), the deposition or the growth of thermal silicon dioxide to produce thick oxide-mask layers

N.C. MacDonald f Microelectronic Engineering 32 (1996) 49-73

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N.C. MacDonald / Microelectronic Engineering 32 (1996) 49-73

(b)

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is difficult and becomes a limiting step for deep etching. Two methods have been developed recently to increase the thickness of the oxide-etch mask for deep silicon etching.

3.4.1. Thermal oxidation of silicon microstructures The first method used to increase the thickness of the oxide-etch mask involves the oxidation of silicon microstructures [34]. Fig. 10, steps (i)-(iv), show the required process steps to produce a thick etch-mask for the subsequent SCREAM process steps (v)-(viii). First, a thin « 1 urn) silicon dioxide mask is deposited or thermally grown on the silicon substrate (Fig. lO(i)). The oxide layer is patterned and the silicon is etched to a nominal depth of 10-20 urn, The silicon structures, typically 0.5-1.5 urn in width, are thermally oxidized from the top and the sides to produce a thick 10-20 urn

s.c. Maclronald / Microelectronic Engineering 32 (1996) 49-73

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high thermally grown silicon oxide etch mask on each silicon microstructure (Fig. lO(iv)). The thick etch mask provides an adequate mask for deep etching (> 100 urn) of the silicon microstructures. Thicker etch masks (>20 J.Lm) can be made using this process without the problems and process time associated with depositing thick layers of thermal silicon dioxide on a planar surface. Another advantage of using thermal oxidation of the SCS beams to produce the etch-mask is the final beam-composite of thermal silicon dioxide (the remaining etch mask) and the SCS microstructure has both good mechanical properties and an excellent insulator on top of the beam. We have recently shown that the fatigue and stability properties of thermally grown silicon oxide [35] are substantially better than CYD-deposited silicon dioxide. 3.4.2. Spin-on-glass etch mask A second method used to produce a thick etch mask for deep etching is the use of Spin-on-Glass (SOG) [36]. Fig. 11 illustrates the process steps (a)-(dt) to produce the thick SOG etch-mask. For the SOG process, silicon trenches (not silicon pillars) form the final pattern. The trenches are filled with spin-on glass; the SOp is cured and the wafer is mechanically or chemical-mechanically polished to planarize the surface. Thus, the SOG-filled trenches form the oxide mask for the subsequent deep-etch step (Fig. 11(d2)). The remaining process steps (Figs. ll(e)-(h)) are the standard SCREAM steps. After the deep-etch step, the SOG can be removed from the structure or left on the structure for the subsequent release step. Unlike, the thermal silicon dioxide masks (Fig. 10), the mechanical and electrical properties of released, composite SOG-SCS microstructures have not been fully characterized.

4. SCREAM MEMS The objective of our research is to build MicroSensors and Microlnstruments that may have up to six degrees of freedom. Such MEMS require a flexible, scaleable process that is applicable to building integrated, and nested large scale (mm-scale) and small scale (u.m-scale and nm-scale) structures. Batch fabrication, array architectures, microelectromechanical integration and microelectronic integration are major themes of our microinstrument research. 4.1. SCREAM attributes for MEMS

The motivational factors for the development and use of the SCREAM high-aspect-ratio, single crystal silicon processes to a build micro instrumentation include the following: {I} A very large vertical or out-of-plane stiffness is required for released SCS beams that span rum-scale to em-scale dimensions. The vertical stiffness scales as (bll)3 and b is the height of the silicon beam and I is the length or span of the beam. The height b is determined by the depth of the silicon anisotropic etch-step and the amount of undercut at the release step. {2} For capacitor sensors and capacitor actuators, the capacitance is proportional to the side-wall area of each plate which is proportional to bl and to the number of plates N (refer to Fig. 1). The force generated by the capacitor actuator is also proportional to b, so large sensing capacitors and high force actuators can be realized with SCREAM-based processing. Force in excess of 100 milliNewtons per

N .C. Maclionald I Microelectronic Engineering 32 (1996) 49-73

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(h)

Fig. 11. A spin-an-g lass (Sa O) process no w used to make a thick etch mask for deep RlE . III step (c) the structure is planarized using chemical-mechanical polishi ng. The structure is then etc hed (d 1); a continuation of the RIE produces very high-aspect-ratio structures which are released using the standard SCREAM steps (e) through ( h).

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cm ' of silicon at 40 Volts is possible using SCREAM processes and capacitive actuators. Thus, large, stiff structures can be displaced 100/-Lm and multi-axis actuators can be nested [32] on a stiff and planar, high-respect-ratio backbone structure (Fig. 2). {3} SCS beams can be thermally oxidized to achieve electrical and thermal isolation along the beam [32] and at the beam anchors [29,32] (Fig. 6). Thermal silicon dioxide exhibits good mechanical properties [35] and excellent electrical isolation. Also, the beams can be coated with CVD insulators (Fig. 8) to produce additional layers of electrical interconnects. Thus, the interconnects are an integrated part of the suspended structures. {4} The aspect-ratio of SCS microstructures can be increased by using different methods [34,36] to form an etch-mask (Figs. 10 and 11). The etch masks for deep silicon etching - large b in Fig. 1 - can be made by the oxidation of silicon-etched structures or by filling silicon trenches with SOG to form the desired microstructures. Fig. 12 plots the curvature (llradius of curvature) of the beams as a function of the height of the beam or the beam aspect-ratio, (b/a) (Fig. 12(b)). For b > 40 urn and a = 1.5 u.m, very planar structures are possible as shown by the curvature approaching zero for large b in Fig. 12. {5} Microstructures with b > 100 urn can be designed to be removed from the silicon substrate [34] and attached to another substrate. Thus SCREAM MEMS with b > 100/-Lm are very robust and can be attached, stacked, and assembled on other substrates and packages. {6} Further improvements in the magnitude of b and the aspect-ratio and spacing of SCREAM microstructures depend on advances in high throughput (optical) lithography; improvements in the pattern transfer capabilities of reactive ion etching processes and equipment; and improvements in new planarization techniques such as chemical-mechanical polishing. All three processes, lithography, reactive ion etching, and chemical-mechanical polishing, follow the technology trends and leadership of the silicon integrated circuit and semiconductor equipment industries - an excellent fast track for MEMS technology advances and improvements. {7} Transistors and diodes can be placed on released and moving SCS beams [31] to provide local

.25 11m RI

.25 11m SI02

SCS 2.5 11 rn

(a)

(b)

Fig. 12. (a) A graph of the planarity or curvature (l/radius of curvature) for a suspended and released SCREAM microstructure. The SCREAM structure consists of a SCS core 1 p.m wide coated layers of silicon dioxide and aluminum. The graph shows that very planar structures are possible for a beam height b > 40 urn. The planarity depends on the thickness of the thin films on the structure. (b) A schematic of the composite SCS beam showing the silicon dioxide and aluminum on the top and sidewalls of the structure.

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amplification, low impedance outputs on released structures, local sensing, and local electronic switching. {8} The SCREAM process incorporates processes to integrate nm-scale tips for field emission [28,32], scanned-probe instrumentation [38] and electron tunneling-based sensors and microinstrumentation. {9} SCREAM-I (Fig. 8) is a low temperature process which allows integration of released and movable structures on prefabricated VLSI wafers [33]. Only a few masking layers are required to produce movable microstructures. {1O} Out-of-plane, movable structures with an area greater than 100 f.LID X 100 f.LID are important for fiber optics-based MEMS. Large, movable, out-of-plane structures can be rotated and translated with up to six degrees of freedom with the addition of torsional microstructures. {II} Torsional actuators [37,38] based on the torsional motion of rectangular cross section, SCS beams provide vertical, out-of-plane motion of silicon structures. Torsional motion of large microstructures perpendicular to plane of the silicon substrate is also possible. The latter motion is important for fiber-optical applications of SCREAM. Very compact, high packing density torsional actuators can provide motion for tips and optically flat mirrors. Repeated thermal oxidation of SCS surfaces reduces surface roughness to produce mirror-like surfaces for micro-optical devices. {12} Thin film sacrificial layer processes [1] can be integrated with SCREAM microstructures and devices to implement more complex MEMS; to selectivity add mass [39] to a SCS microstructure; to form compact flexures and springs; to make mirrors and interferometers; to form electrical components (resistors); and to form catalytic and chemically active layers.

4.2. SCREAM MEMS examples: Microsensors and microinstruments Many examples of SCREAM devices and microinstruments have been fabricated and characterized including "a microelectromechanics-based artificial cochlea [40]; a "microtribology machine" [41]; "latching snap fasteners for micro assembly" [42]; "capacitance-based tunable micromechanical resonators" [43,44]; tunable, mm-wave resonators and waveguides [45]; micro-optical-mechanical devices [46,47], and a motion amplifier [48]. Two SCREAM microinstruments: the Milli-Newton Micro-loading Device [49] and the Micro Scanning Tunneling Microscopes [38] are briefly described to illustrate a few of the key features of the SCREAM processes for making microinstruments. A subset of the twelve design and process factors listed above ({1}-{12}) are referenced for each micro instrument to highlight the issues most important to its design and operation.

4.2.1. Micro loading machine Loading machines are used to apply forces to macroscopic structures to characterize the static and dynamic properties of structures, to characterize structural failure modes, and to determine the fatigue properties of the structures. Because such loading instruments are large, it is difficult to use these instruments to characterize micrometer-scale and nm-scale structures. One major difficulty is the handling and the attaching of microstructures to such large machines. A second difficulty is verifying that loading measurements obtained on the large loading machines reflect the structural characteristics of the integrated microstructures and rnicrosupports on the actual MEMS. To circumvent these measurement difficulties, we have developed micro-electromechanical loading machines [49] and

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other micro-instruments to characterize the mechanical, frictional [41], and electrical properties of microstructures and thin film materials. The micromachines are made using the SCREAM process, so the integrated microstructures, microsupports and thin films under test are identical to those structures used to fabricate the actual MEMS. The rnicroloading machine [49] shown in Fig. 13(a) occupies a "silicon chip area" of 4 mm X 5 mm and uses integrated comb drive capacitor actuators (Fig. l(a)-A) to generate the required loading force. Nine thousand comb drive fingers are structurally connected in parallel on a stiff, SCS backbone structure shown in Fig. 2 and in schematic form in Fig. 13(b). The comb fingers are 15 J.Lm long with a pitch of 7 J.Lm. The device consists of 1-2 J.Lm wide and 12 ILm deep SCS beams. The overlap between the movable and the fixed combs is 7 J.Lm, and the spacing between the movable and fixed comb fingers is nominally 1.5 J.Lm. All beams, cross bars, and combs have the same nominal cross section shown in Fig. 2 with b = 12 J.Lm. The microloading machine applies a compressive force of 2 milli-Newtons quasi-statically by electrically actuating (56 Volts) the 9000 comb capacitors. The same capacitors or a subset of the capacitors can be used to sense the displacement of the device by recording the change of capacitance. The functionality of the loading device is demonstrated by compressing two slender SCS bars (A-B and C-D) and buckling them (Fig. l4(a». Only 2000 of the 9000 comb-fingers are used to generate the force necessary to buckle the beams. Fig. 14(b) shows the deflection amplitudes of the two beams as a function of the voltage applied to the 2000 comb-fingers. With an increase of voltage, the fixed-fixed beam A-B gradually buckles. Then, beam C-D, which is free at end D and is initially separated from the wall by 0.5 urn, touches the wall and begins to buckle. Beams A-B and C-D have identical cross sections. The results of the buckling experiment are used to calibrate the load generated by the device and the spring constant of the device. Note that the beam A-B does not have any appreciable deformation until about 20 Volts (Fig. l4(b». Above 20 volts the transverse displacement increases at a fast rate until C-D touches the wall and starts sharing the compressive force at about 30 volts. Consequently, the deformation rate of A-B decreases until C-D also buckles and then both the beams deform at a similar rate. The loading machine is calibrated by relating the theoretical forces required to buckle a fixed-fixed beam, A-B, and to buckle a fixed-hinged beam, C-D, with the Wing Arm li mb

Comb

~/

Specimen 5mm

area ,

Fig. 13. (a) Micrograph of a microloading machine on a "silicon chip". The machine spans an area of 4 mm X 5 mm, and supports 9000 capacitative actuators on a backbone structure. The microactuator applies pressure on the two SCS beams (A-B and C-D) under test. (b) A schematic showing the design of the microloading machine.

67

N.C. MacDonald / Microelectronic Engineering 32 (/996) 49-73 20.0

V

15.0

!

~

'I 1$ Q

>J/;V meAll

FIx d-Fba Bel

10.0 5.0 0.0

!

/

/

~

ed-Hi

r.......

i'Mm pO)

-5.0

-10.0 0.0 10.0 20.0 30.0 4Q.0 50.0 80.0 Voltage (V)

Fig. 14. (a) Two SCS beams buckling under load applied by the microloading machine. Beam A-B is a fixed-fixed beam. Beam C-D is a fixed-hinged (D) beam and shows non-zero slope at D. (b) A graph of the deflection of the buckling SCS beams as a function of the voltage applied to the capacitive actuators. The fixed-fixed beam (A-B) buckles at V= 30 volts and the fixed-hinged beam (C-D) touches the wall at V=3 5 volts and buckles at V=42 volts.

measured forces (voltages) required to buckle the beams. From these measurements, we can determine the actual stiffness (k = 157 N 1m) of the suspended structure and the aspect-ratio, (bla) = 7.5, of the released SCS beams [49]. The microloading machine can apply a stress of 100 GPa on a 100 nm X 100 nm beam. During the fabrication of the machine, the SCS beams can be replaced with other test specimens including thin film materials and microstructures and nm-scale tips. Furthermore, these small instruments can be attached and operated in large analytical instruments including a Transmission Electron Microscope (TEM), a Scanning Electron Microscope (SEM), Scanning Auger Microprobe (SAM), Scanning Tunneling Microscope (STM) or in low temperature instruments. Thus, these micro instruments should provide access to many new measurements to characterize the electrical, thermal and mechanical properties of um-scale and nm-scale materials, structures, and devices. The loading device is an example of a SCREAM device that demonstrates the importance of stiff, planar (Fig. 12(a)) microstructures [50] - {I}, a stiff backbone support structure (Fig. 13(b)), and parallel operation of actuators - {2} and Fig. 2. In the design of MEMS thermal distortion of composite beams must be addressed [51]. The released micro loading device was subjected to and survived an 1100°C temperature cycle, fluid forces, and centrifuge. The device is highly planar, is stable during actuation, and survives the forces of liquids after structure release. The magnitude of the applied force can be increased by increasing the aspect-ratio bla of the structures {2, 4, 6}. 4.2.2. Micro-scanning tunneling microscope Scanned-probe instruments, including the Scanning Tunneling Microscope (STM) and the Atomic Force Microscope (AFM), have developed rapidly over the past decade. Two recent reviews highlight the design, methods, and applications of STMs and AFMs [52,53]. The importance of these instruments lies in their ability to image and manipulate single atoms, to measure forces on the atomic scale, and to perform nm-scale lithography [54-57]. Most scanned-probe instruments use large piezoelectric actuators for the precise positioning of a tip or a probe in three dimensions (x, y, z), The size of the actuator limits the performance of the instrument in two ways. First, the size and mass of the scanner result in a slow scan rate (1-100 Hz). Second, and more importantly, these instruments

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can not be easily integrated into arrays of massively parallel scanned-probe devices that are required for high speed atom manipulation and information storage, and for high throughput, nm-scale lithography systems. Two Micro-Scanning Tunneling Microscopes (Micro-STMs) with integrated nm-scale tips were fabricated using the SCREAM process [38]. One Micro-S'TM shown in Fig. I5(a) measures approximately 200 urn on-side and is an example of an STM element for a "micro-STM-arrayarchitecture". Another, larger Micro-STM/AFM shown in Fig. 16(a) measures 2 mm on-a-side including a 1 mm long cantilever. At the end of the cantilever is a 20 nm diameter integrated tip on a 6 urn high by I!-Lm diameter support shaft as shown in Fig. I5(b). Both versions of the Micro-S'TM use released SCS comb capacitors - (Figs. 1 and 2) and {2} - as drives for scanning the integrated tip in x and y. The z motion for the integrated tip - {8} - is produced with a torsional actuator - refer to Figs. 15(a) and 16, and {II}. The larger Micro-S'TM (Fig. I6(a)) includes SCS specimen-supports around the periphery of the device to support the sample to be imaged. The Micro-S'I'M was tested by imaging a 300 nm wide metal conductor on a silicon chip placed on the silicon specimen-supports. The support beams which extend from the anchor posts at the periphery of the Micro-S'TMs serve as the restoring springs for the scanning stage. As shown in Fig. I5(a), the 200!-Lm on-a-side Micro-S'TM has four comb capacitor drives to move the center stage in the +x, -x, +y, and -y directions. The nominal displacement generated by each drive is about 20 nm with 25 Volts applied. The stiffness, k, in both x and y is about 76 N/ m, and the frequency response in x and y is on the order of 1 MHz. For the 2 mm on-a-side Micro-S'TM (Fig. 16), three groups of comb capacitor drives are used to move the released stage in the +x, +y and -y directions. Out-of-plane motion is achieved with an integrated torsional z-drive. In Fig. I5(a), the torsional z-drive is comprised of a torsion bar across the center of the rectangular stage. Two metal pads underneath the torsional stage form parallel plate capacitor drives with the SCS beams of the stage. When a voltage is applied to an electrode on one side of the stage, that side of the stage is pulled downward and the opposite side moves upward, similar to the motion of a teeter-totter. The high aspect ratio tunneling tip is the bright spot located at the center of the lower outer-most beam of the

(b)

Fig. 15. (a) A micro-scanning tunneling microscope or micro-S'TM, The "comb-like" structures are the :!::x - y microactuators. The structure in the center is a torsional z microactuator that moves the paddle-like structures out-of-plane of the silicon. (b) Detail of the paddle-like structure showing the integrated silicon tip (20 nm diameter) on a SCS post.

N.C . Maclronald I Microelectronic Engineering 32 (1996) 49-73 (a)

69

(b)

( 295 nm

[0nm 200 nm

(c)

Fig. 16. (a) A SEM micrograph of a larger - a few mm - on-a-side micro-S'I'M. The SCREAM-fabricated micro-STM includes xyz micro actuators and a torsional cantilever beam with an integrated tunneling tip. Capacitive microactuators move the tips in x - y - z. The two square SCS structures at the left of the micrograph are used to support the sample which is placed on top of the micro-STM. (b) A Scanning Tunneling Microscope (STM) image obtained w ith the micro-STM shown in (a). The "silicon chip" sample is placed on-top-of the micro STM and is supported by the SCS posts shown in Cal. (c) A SEM image of the 300 nm T iW lines.

rectangular stage and is made by the thermal oxidati on of silicon [38]. After thermal oxidation, a cap similar to that in Ref. [28] remains on the tip to protect the tip during subsequent processing. In Fig. 16(a), the torsional z-drive consists of a released square truss or plate on the right of the torsion bar, and a 1 mm long cantilever on the left of the torsion bar. The electrodes patterned underneath the square plate and the cantilever form two parallel plate capacitor drives to generate motions in the +z and - z directions respectively. The high aspect ratio tunneling tip is located on the far left end of the cantilever. Two electrical isolations - {3} - using therm al oxidation are required for interconnections. A 150 nm aluminum film is depo sited to make the drive electrodes, as well as the interconnects. Fig. 15(b) shows the integration of the high aspect ratio tip with the torsional z drive. Fig . 16(b) shows the STM image obtained with the larger Micro-STM. This picture shows an area about 200 nm by 200 nm near the edge of a groove on the test sample. Fig. 16(c) is an SEM micrograph of the 300 nm Au/Pd metal lines patterned using electron beam lithography. The two Micro-S'TMs are examples of the integration of three dimensional actuation with nm-scale tunneling tips. The Micro-S'TMs illustrate the need for the integration of electrical isolation and

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interconnects on suspended, moving microstructures. One Micro-STM (Fig. 15) is an example of a STM element for an array architecture. Massively parallel arrays of such devices have potential applications for terabit information storage "on-a-chip", nm-scale machining, high throughput, nm-scale lithography, and robots for manipulating atoms.

5. Summary The SCREAM process has been developed to fabricate very high-aspect ratio , suspended silicon structures over large areas - 1 ern x 1 ern. Deep silicon etching usually requires thick etch masks which are deposited or grown and patterned on the silicon substrate. For SCS structures of 1 to 20 urn deep, CVD deposited oxide or thermally grown oxide can be used as the etch masks. For very deep etching (> 30 urn), robust oxide etch masks can be produced by thermal oxidation of patterned silicon microstructures or SaG filling of silicon trenches that delineate the desired microstructures. Deep etching of the patterned oxide microstructures generates SCS beams topped with an oxide layer- the remaining mask layer. The integration of micrometer-scale electrical (thermal) isolation and micrometer-scale contacts are key attributes of the SCREAM process. Complex integrated MEMS have been fabricated using this process, including nested X-Y-Z actuators and other complex MEM microstructures that require a stiff, high-aspect-ratio SCS backbone structure and nested, high force microactuators . A low temperature version of SCREAM called SCREAM-I, eliminates a critical second mask-etch Figs. 7(f) and (g) step and a third metal mask. The metal deposition is the last step in the process. The low temperature nature of SCREAM-I allows the fabricating of released structures on completed wafers with integrated circuits [33]. Two examples of SCREAM-based microinstruments were described to highlight the importance of high-aspect-ratio SCS microstructures for MEMS. The importance of achieving a very large, vertical or out-of-plane stiffness required to produce planar, released SCS beams and to produce a high force actuator array that spans mm-scale dimensions was illustrated in the description of the Milli-Newton Micro-loading Device [49]. It was also noted that the vertical stiffness scales as (bll)3 where b is the height of the silicon beam and l is the length or span of the beam. The height b is determined by the depth of the silicon anisotropic etch-step and the amount of undercut at the release step. For capacitor sensors and capacitor actuators, the capacitance is proportional to the side-wall area of each plate which is proportional to bl and the number of plates N (Fig. l(a)-A). The force of the capacitor actuator is also proportional to b, so large sensing capacitors and high force actuators can be realized 2 with SCREAM-based processing. Forces in excess of 100 milliNewtons per cm of silicon at 40 Volts are possible using SCREAM processes and capacitive actuators. Thus, large, stiff structures can be moved 100 urn, and actuators can be nested on a stiff, high-aspect-ratio backbone structure. Also the importance of a compact, torsional actuator to produce vertical or z displacement of an integrated, nm-scale, silicon tip for the Micro-STM [38] was emphasized. In general, both small and large structures need to be integrated into the same microinstrument so the concept of maintaining minimum feature size and minimum microstructure spacing over large mm-scale spans is the key to generating large forces, large sense capacitances, and excellent isolation and separation of mechanical modes. The use of micro instruments with macroscopic analytical instruments including TEM, SEM, SAM

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[33,37] and STM offers new possibilities for um-scale and nm-scale materials, structures and device research. The use of SCS for MEMS allows for the integration of electron devices [31,33] and circuits on the moving structures. The integration of active devices on moving silicon structures should become more prevalent in the sensor and microelectronics fields as we begin to fabricate more complex MEMS and begin to implement MEMS-based array-architectures. Finally, MEMS packaging is a major challenge particularly when the device must transfer forces and displacements to the external environment. A final few comments are in order. High force - say 1 roN to I N - MEMS design offers many opportunities to increase the frequency response, bandwidth and dynamic range; to perform six degrees of freedom positioning; to isolate mechanical modes; to produce mm-scale displacements; and to manipulate macroscopic objects in array architectures. Such integrated, high force actuators portend a new wave of MEMS.

Acknowledgements This work is supported by ARPA and the National Science Foundation. All fabrication was performed at the Cornell Nanofabrication Facility at Cornell University which is supported by the NSF, Cornell University, and Industrial Affiliates.

References [I] J'w. Gardner, Microsensors Principles and Applications, John Wiley and Sons Ltd., Chichester, UK, 1994. [2] C. Linden, L. Paratte, M.-A. Gretillat, V:P. Jaecklin and N.F. DeRooij, Surface Micromachining, J. Mircomech.

Microeng. 2(3) (1992) 122-132. [3] M. Mehregany and YC. Tai, Surface micro machined mechanisms and micromotors, J. Micromech. Microeng, 1(2) (1991) 73-85. [4] K.E. Bean, Anisotropic etching of silicon, IEEE Trans. Electron Devices 25 (1978) 1185-1192. [5] W. Kern, Chemical etching of silicon, germanium and gallium phosphate, RCA Review 39, pp. 278-307. [6] J.S. Danel and G. Delpierre, Quartz: A material for microdevices, J. Micromech. Microeng. 1(4) (1991) 187-198. [7] C.E. Hunt, C.A. Desmond, D.R. Ciarlo and W,J. Bennett, Direct bonding of micromachined silicon wafers for laser diode heat exchanger applications, J. Michromech. Microeng, 1(3) (1991) 152-156. [8] Y.B. Gianchandani and K. Najafi, A bulk dissolved wafer process for microelectromechanical devices, J. Microelectromechanical Systems 1(2) (1992) 77-85. [9] H. Guckel, KJ. Skrobis, T.R. Christenson, 1. Klein, S. Han, B. Choi, E.G. Lovell and T.W. Chapman, Fabrication and testing of the planar magnetic micromotor, 1. Micromech. Microeng. 1(3) (1991) 135-138. [10] W. Ehrfeld, F. Gotze, D. Miinchmeyer, W. ScheIb and D. Schmidt, 1988 LIGA Process: Sensor construction techniques via x-ray lithography, in: IEEE Solid-State Sensor ana Actuator Workshop, Hilton Head, SC, IEEE, New York, 1988, pp. 1-4. [11] S.M. Sze, Semiconductor Sensors, John Wiley and Sons, Inc., New York, 1994. [12] H. Reichl, Editor, Sensors Actuators System Integration, Microsystem Technologies 1(1) (1994). [13] W.N. Carr and H. Guckel, Editors-in-Chief, Structures, Devices and Systems, Micromechanics Microengineering 1(1) (1991). [14] S. Middelhoek, Editor-in-Chief, A Special Issue Devoted to Micromechanics, Sensors and Actuators 20(1&2) (1989). [15] W. Trimmer, Editor-in-Chief, A Joint IEEE/ASME Publication on Microstructures, Microactuators, Microsensors and Microsystems, Microelectromechanical Systems 1( I) (1992).

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[16J Solid-State Sensor and Actuator Technical Digest, Sponsored by the Transducers Research Foundation, currently six proceedings (published in even-numbered years from 1984). [17J IEEE Micro Electro Mechanical Systems MEMS Proceedings, An Investigation of Micro Structures, Sensors, Actuators, Machines and Systems, currently eight proceedings (published in odd-numbered years from 1981). [18] Transducers International Conference on Solid-State Sensors and Actuators Digest of Technical Papers, currently eight conference proceedings (published in odd-numbered years from 1981). [19J KE. Peterson, Silicon as a mechanical material, Proc, IEEE 70 (1982) 420-457. [20J K Hjort, J. Soderkvist and J.-A. Schweitz, Gallium arsenide as a mechanical material, J. Micromech. Microeng, 4(1) (1994) 1-13. [21] W.N. CUlT, Editor-in-Chief, Special Issue on Microfluidics, Micromechanics Microengineering 4(4) (1994); also, P. Gravesen, J. Branebjerg and O.S. Jensen, Microftuidics-A review, J. Micromech. Microeng. 3(4) (1993) 168-182. [22J K.J. Gabriel, Engineering microscopic machines, Scientific American 273(3) (1995) 150-153. [23] Z.L. Zhang and N.C. MacDonald, An RIE process for subrnicron, silicon electromechanical structures, J. Micromech. Microeng, 2(1) (1992) 31-38. [24] K.A. Shaw, Z.L. Zhang and N.C. MacDonald, SCREAM I: A single mask, single-crystal silicon, reactive ion etching process for microelectromechanical structures, Sensors and Actuators A 40 (1994) 63-70. [25] Z.L. Zhang and N.C. MacDonald, Fabrication of submicron high-aspect-ratio GaAs actuators, J. MEMS 2(2) (1993) 66-72. [26] W.e. Tang, T.R. Nguyen and R.T. Howe, Laterally driven polysilicon resonant microstructures, Sensors and Actuators 20 (1989) 25-32. [27] S. Arney and N.C. MacDonald, Formation of submicron silicon on insulator structures by lateral oxidation of substrate-silicon islands, J. Vac. Sci. Technol. B 6(1) (1988) 341-345. [28] J.P. Spall as and N.C. MacDonald, Self-aligned silicon field emission cathode arrays formed by selective, lateral thermal oxidation of silicon J. VaL'. Sci. Technol. B 11(2) (1993) 437-440. [29] J. Yao, S. Arney and Noel C. MacDonald, Fabrication of high frequency two-dimensional nanoactuators for scanned probe devices, J. Mlcroelectromechanical Systems 1(1) (1992) 14-22. [30J J.J. Yao and N.C. MacDonald, A micromachined, single crystal silicon tunable resonator, J. Micromech. Microeng, 5(3) (1995) 257-264. [31] J.J. Yao, S.C. Arney and N.C. MacDonald, A fully-suspended, movable, single crystal silicon, deep submicron MOSFET for nanoelectromechanical applications, Sensors and Actuators A 40 (1994) 77-84. [32] Z. Lisa Zhang and N.C. MacDonald, Integrated silicon process for micro-dynamic vacuum field emission cathodes, J. Vac. Sci. Technol, B 4(6) (1993) 2538-2543. [33J KA. Shaw, S.G. Adams, F.M. Bertsch, P.G. Hartwell and N.C. MacDonald, Integrating SCREAM micromachined devices with integrated circuits, in: Ninth lnternat. IEEE Workshop Oil MEMS, February II-IS, 1996, San Diego, CA, USA. [34] A. Jazairy and N.C. MacDonald, Very high aspect ratio wafer-free silicon micromechanical structures, in: T. Postek, ed., SPIE's 1995 Symposium on Microlithography and Metrology in Micromachining, Austin, TX, Proc. SPIE 2640, pp. 11l-120. [35] 1. Ogo and N.C. MacDonald, Micromechanical structures for electron and ion beam irradiation phenomena, J. VaL'. Sci. Technol. B 12(6) (1994) 3285-3288. [36] X.T. Huang, L.-Y. Chen and N.C. MacDonald, A low temperature process for very high aspect ratio silicon microstructures using SOG etch mask, in: M.T. Postek, ed., SPIE's 1995 Symposium on Microlithography and Metro logy in Micromachining, Austin, TX, Proc. SPlE 2640, pp. 178-183. [37] R.E. Mihailovich and N.C. MacDonald, Dissipation measurements of vacuum-operated single crystal silicon microresonators, Sensors and Actuators 50 (1995) 199-207. [38] Y. Xu, S.A. Miller and N.C. MacDonald, Integrated micro-scanning tunneling microscope, Appl. Physics Lett. 67(16) (1995) 2305-2307. [39] N.C. MacDonald, L.Y. Chen, J.J. Yao, Z.L. Zhang, J.A. McMillan, D.C. Thomas and K.R. Haselton, Selective chemical vapor deposition of Tungsten for microelectromechanical structures, Sensors and Actuators 20 (1989) 123-133. [40] D. Haronian and N.C. MacDonald, A microelectromechanics-based artificial cochlea (MEMBAC), in: Transducers' 95, Stockholm, Sweden, Vol. 2, pp. 708-71 I.

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[41] R. Prasad and N.C. MacDonald, Design, fabrication and measurements of friction in SCREAM micro-devices, in: Transducers '95, Stockholm, Sweden, Vol. 2, pp. 52-55. [42] R. Prasad, K.F. Bohringer and N.C. MacDonald, Design fabrication and characterization of single crystal silicon (SCS) latching snap fasteners for micro assembly, in: i995 lnternat, Mechanical Engineering Congress & Exposition (iMECE '95); Symposium on Micro-Mechanical Systems, San Francisco, CA. [43] S. Adams and N.C. MacDonald, Capacitance-based tunable micrornechanical resonators, in: Transducers '95, Stockholm, Sweden, Vol. I, pp. 438-441. [44] S. Adams, F. Bertsch and N.C. MacDonald, Independent tuning of the linear and nonlinear stiffness coefficients of a micromechanical device, in: Ninth Internat. iEEE Workshop on MEMS, February 11-15, 1996, San Diego, CA, USA. [45] A.A. Ayon, N.J. Kolias and N.C. MacDonald, Tunable, micro machined parallel plate resonators and transmission lines, in: Proc. Asia-Pacific Microwave Conf., Taejon, Korea, 1995. [46] N.C. MacDonald and A. Jazairy, Single crystal silicon: Application to micro-opto-electromechanical devices, in: SPIE Micro-Optics/Micromechanics and Laser Scanning and Shaping, San Jose, CA, Proc. SPIE 2383, pp. 125-135. [47] A. Jazairy and N.C. MacDonald, Very high aspect ratio smooth silicon mirrors for optical microresonators, J. Vac. Sci. Technol. B (May [June 1996), to appear. [48] X.T. Huang and N.C. MacDonald, Micromotion Amplifier, in: Ninth International IEEE Workshop on MEMS, February II-IS, 1996, San Diego, Ca, USA. [49] M.T.A. Saif and N.C. MacDonald, A mill i-Newton micro loading device, in: Transducers '95, Stockholm, Sweden, Vol. 2, pp. 60-63. [50] M.T.A. Saif and N.C. MacDonald, Design Considerations for MEMS, in: 1995 North American Conference on Smart Structures and Materials, San Diego, CA, SPIE Proc. 2448, pp. 93-104. [51] M.T.A. Saif and N.C. MacDonald, Thermal Stresses in Composite MEMS, in: 1995 North American Conference on Smart Structures and Materials, San Diego, CA, SPJE Proc. 2441, pp. 329-340. [52] R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge, 1994. [53] J.A. Stroscio and W.J. Kaiser, eds., Scanning Tunneling Microscopy, edited by J.A. Stroscio and W.J. Kaiser, Academic Press, Boston, MA, 1993. [54] G. Binnig and H. Rohrer, Scanning tunneling microscopy, Helvetica Pliysica Acta 55 (1982) 726-735. [55] G. Binnig, C.F. Quate and Ch. Gerber, Atomic Force Microscope, Phys. Rev. Lett. 56 (1986) 930-933. [56] D.M. Eigler and E.K. Schweizer, Positioning single atoms with a scanning tunneling microscope, Nature 344 (1990) 524-526. [57] C.R.K. Martian, ed., Technology of Proximal Probe Lithography, SPIE, Bellingham, WA, 1993.

N.C. MacDonald received the Ph.D. degree in Electrical Engineering from the University of California at Berkeley in 1967. Currently, he is a Professor in the School of Electrical Engineering and the Director of the Cornell Nanofabrication Facility at Cornell University, Ithaca, NY. He WaS Director (Chair) of the School of Electrical Engineering ('89-'94). Dr. MacDonald was in industry (1968-1983) first as an entrepreneur, engineer and manager, including Division General Manager, in Physical Electronics Industries, Inc. and in Perkin-Elmer Corporation. Dr. MacDonald has specialized in electron beam technology and has been instrumental in combining Auger Electron Spectroscopy with Scanning Electron Microscopy and the development and commercialization of the Scanning Auger Microprobe. His present interests include MicroElectroMechanical (MEMS) with emphasis on microinstruments and massively parallel nm-scale information storage, lithography and molecular-scale manipulation. Dr. MacDonald received the 1973 Victor Macres Memorial Award and 1975 Young Engineer of the Year Award. He is an IEEE Fellow.