Chemical Sensing With Micromolded Plastic ... - IEEE Xplore

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 6, DECEMBER 2005

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Chemical Sensing With Micromolded Plastic Microcantilevers Andrew W. McFarland and Jonathan S. Colton, Fellow, ASME

Abstract—This paper describes microcantilever sensors produced via injection molding. The injection mold design is novel in that it employs one floating and one fixed mold half, hence only necessitating high flatness on two surfaces (e.g., the mating surfaces of the mold), whereas the remainder of the mold can be machined to only moderate tolerances. The mold holds a sub-100 nanometer flatness error over the entire mold mating surfaces, needed to produce micro- and nanoscale parts. Micrometer-scale cantilevers are produced and characterized as a test case. Microcantilevers are fabricated from three different polymeric materials and have exceptional repeatability as evidenced by their measured first-mode bending resonant frequencies. As a precursor to biological sensing, gold-thiol chemical sensing results obtained with the injection-molded cantilevers are also presented and show values that agree with the literature. As a whole, this work shows that the polymeric microcantilever parts fabricated via injection molding are mechanical and functional equivalents to their silicon-type counterparts, and are cheaper and easier to manufacture. [1483] Index Terms—Microfabrication, micromolding, nanomolding. Fig. 1. Description of the injection molding process.

I. INTRODUCTION

U

NDOUBTEDLY, the microfabrication field has grown due to the rise in computing power, and especially so because of the transistor developed by Shockley, Bardeen, and Brattain at Bell Labs in 1947, and its later refinement by Bardeen and Brattain [1]. What followed was an explosion of transistor usage (vis-à-vis the exponential predictions of Moore’s Law [2]) and along with it, rapid advancement of the technologies used to produce transistors. Petersen’s seminal paper on the use of silicon as a true engineering material showed that, in addition to transistors and circuits, silicon-based micron-scale sensors, actuators, and, indeed, microsystems were possible [3]. While the expanse and functionality of the silicon-based microsystems and parts is vast and impressive, the advantages of other materials and processes have not been overlooked in the microfabrication realm. Rötting et al. gave a comprehensive description of many available micro-part fabrication techniques [4]. Of interest in this work is microfabrication of thermoplastic structures, and five main processes have been employed for this: injection molding, reaction injection molding, hot embossing, injection compression molding, and thermoforming [5]. Particularly, this work involves thermoplastic injection molding (IM) at the micron scale, or micromolding, for MEMS applications. Manuscript received December 10, 2004; revised March 17, 2005. This work was supported by NIH Grant EB 000767. Subject Editor C. Liu. The authors are with the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Manufacturing Research Center (MaRC), Atlanta, GA 30332-0405 USA (e-mail: jonathan.colton@ me.gatech.edu). Digital Object Identifier 10.1109/JMEMS.2005.851853

The growth of micromolding can be judged by the recent influx (within the last ten years or so) of commercially-available injection molding machines geared at the molding of small-volume parts (masses of roughly 0.1–0.001 grams). Such machines are produced by many companies (e.g., Rondol, Medical Murray, Battenfeld, Nissei, and Boy [6], [7]). The injection molding process is depicted in Fig. 1. The portions of the molding machine that form the mold cavity itself are the top and bottom inserts. The injection stage first clamps the mold inserts together (the black arrows on the left side of Fig. 1 depict the clamping forces acting on the mold inserts) and then injects a polymer melt into the cavity formed by the mold inserts. The holding stage continues to apply pressure to the melt after the cavity has filled (to minimize thermal shrinkage). The cooling stage begins once certain portions (or all) of the polymer melt have solidified and continues for a predetermined time to allow for further cooling until the ejection stage begins, when the mold halves are opened and the part(s) is (are) removed. Worldwide, many research groups are active in the micromolding field, e.g., the Institut für Mikrotechnik Mainz GmbH, Institut für Kunststoffverarbeitung (IKV), the Ohio State University, the University of Cincinnati, the University of California at Berkeley, and the Georgia Institute of Technology [5], [8]–[14]. Some researchers have employed conventional machining at the micron scale to produce metal molds for injection molding and analyzed such processes [14]–[18]. Electrochemical machining (ECM [19]), electrodischarge machining (EDM [20]), and focused ion beam (FIB [21]) techniques are

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and added masses stress change on the order of on the order of femtograms [36], [37]. Numerous motion detection schemes exist to monitor the deflection and resonance behavior of a microcantilever and these include tunneling current tracking, the optical lever, interferometry, and piezoresistive approaches [38]–[41]. The most common approach (and that used exclusively in this work) is the optical lever, in which a laser is reflected off one surface of a microcantilever into a photodiode. The distribution of the laser energy upon the photodiode coupled with the known machine geometry allows for determination of microcantilever deflection and resonance behavior [39]—this approach yields atomic-scale resolution [42]. The most common microcantilever materials used are silicon, silicon-nitride, gallium-arsenide, and occasionally diamond [43]. Fig. 2.

Generic base part with six protruding microcantilevers, scale: — =

100 m.

also feasible for fabrication of metal micromolds. Many groups employ the LIGA technique (for mold production), which consists of lithographically micromachining a silicon-based master, coating the master with a metal, and then removing the silicon substrate leaving a metal part to be used as a micromold [22]. The LIGA process has been used to create very precise and highaspect ratio micron-scale parts [17], [23], [24] but is limited to 2.5D geometries. Others have employed a hybrid molding setup using a micromachined silicon insert and a conventional metal mold support structure [13]. Regardless of the mold production scheme, injection molding is truly an alternative to cleanroombased MEMS fabrication for certain applications as evidenced by the array of functional polymeric parts produced such as: microcantilevers for atomic force microscopy (scanning probe applications) [25], micropumps [8], [26], and biosensing (DNA) microfluidic devices [27]. A. Microcantilever Background Information This paper is focused on the injection mold design used for and the production of polymeric microcantilevers, and the first use of injection-molded thermoplastic polymeric microcantilever parts as chemical sensors. A generic microcantilever part is shown in Fig. 2, detailing the base part and the six protruding microcantilevers (the number of cantilevers was chosen arbitrarily and usually spans from one to eight). The length, width, and thickness of the microcantilevers are roughly , respectively. Microcantilevers are used 500, 100, and 10 as sensors in numerous fields, e.g., chemical sensing [28], calorimetry [29], force spectroscopy [30], and rheology [31]. The reader may consult the literature for further information on microcantilevers [29], [32]. Microcantilevers produced using integrated-circuit (IC) techniques are ubiquitous in atomic force microscope (AFM) and microsensing circles, and are commercially available at costs of $5 to $100, or even more, depending upon application [33], [34]. Two operational modes exist—one to measure the static deflection (to determine surface stresses, for example) and the other to monitor microcantilever resonance frequency behavior (to determine the amount of mass adsorbed by a microcantilever, for example) [29], [31], [35]. Microcantilevers can detect a surface

B. Polymeric Microcantilevers Of more relevance to this work is polymeric microcantilevers, and Genolet and coworkers, among the first active in this field, have produced scanning probe microcantilevers from a photopolymer (SU-8). Their approach was to etch a mold in a piece of silicon, fill the mold with SU-8, photocure the SU-8, and attach a base part for mounting in an AFM. The resulting parts proved to be feasible for obtaining images of DNA and a Langmuir–Blodgett film with coexisting hydrocarbon and fluorocarbon molecular domains—multiple cantilever parts also were made [44], [45]. More recently, Wang et al. also created microcantilevers (SPM probes) using a photopolymer (polyimide) with elastomeric tips [46] using nearly the same technique as Genolet et al. Using IC techniques Thaysen et al. made photopolymer-based microcantilever sensors with integrated piezoresistive elements (for deflection sensing) for use in sensing and surface topography mapping applications [47]–[49]. Lee et al. produced microcantilever arrays from fluoropolymers by using photolithograpy to produce a pattern on a polymeric substrate and then selectively removing material using normal and oblique ion beam etching [50]. While these works are important as they have produced polymeric microcantilevers, their reliance on IC fabrication techniques is seen as limiting in the view of the authors as they are expensive, limited in feasible materials, and very cost-sensitive to design modification. Polymeric microcantilevers fabricated via injection molding offer numerous advantages as compared to silicon-type parts including: the availability of numerous plastic materials as opposed to a limited number of ceramics and photopolymers for the IC-based techniques, greater material property variety (e.g., elastic modulus), simpler chemical functionalization in certain biosensing applications (e.g., antigens or antibodies will bond spontaneously to polystyrene microcantilevers upon incubation), and the ability to be manufactured with arbitrary cross-sectional geometries, which could reduce fluid flow-induced noise in certain applications. Additionally, there could be reduced initial setup and final part costs (assuming sufficient market demand) for injection-molded microcantilevers. It should be emphasized that this work is relevant to applications using “diving board” type microcantilevers (e.g., chemical and biological sensing) and that the development of tipped cantilevers for use in tribology and force spectroscopy applications

MCFARLAND AND COLTON: CHEMICAL SENSING WITH MICROMOLDED PLASTIC MICROCANTILEVERS

(e.g., phonograph-type surface topography mapping [30]) via injection molding is reported elsewhere [25]. It is not meant to be implied and the reader should not infer that polymeric microcantilevers as described in this paper are feasible for every application where silicon microcantilever parts are, nor vice versa. To date, injection molding has proven feasible for production of microcantilevers from a single polymer—polystyrene [14]—and the minimum thickness was shown to be roughly 2 with length/thickness ratios (i.e., aspect ratios) over 170. However, no work has produced cantilevers from numerous polymeric materials in a repeatable fashion and, more importantly, no work has proven that cantilevers produced from any of the three non-IC techniques (e.g., film cutting [51], solvent casting [52], and injection molding [14]) are repeatable and accurate chemical sensors. This work sought to remedy both of these shortcomings, and did so via repeatable injection molding of microcantilevers from three different classes of polymeric materials and performing chemical sensing with a subset of the produced parts showing results commensurate with literature values [53]. The paper comprises two topics—the repeatable fabrication of the microcantilevers via injection molding and the chemical sensing performed.

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Fig. 3. Exploded view of injection mold setup.

II. INJECTION MOLDING EXPERIMENTAL SETUP A. Mold Support Structure Design Due to the decreased part size in micromolding, the magnitude of the mold manufacturing tolerances diminish significantly compared to conventional-scale molding. While it is relatively simple to reduce the size of the mold cavity itself, it is much more difficult to satisfy reduced manufacturing tolerances over the entire mold. Therefore, a new injection mold design methodology was adopted for this work which consisted of separating the portions of the mold which require extremely high manufacturing tolerances from those which do not. To accomplish this decoupling, a fixed-floating molding apparatus was designed. One half of the mold is supported by a spring-loaded, floating support structure and mates with the other half of the mold which is rigidly mounted. Fig. 3 shows an exploded view of the mold, and Fig. 4 shows a collapsed view. As a result of this design, the stringent flatness and surface roughness tolerances need to be satisfied by only the two mold insert surfaces which meet at the parting plane. B. Mold Inserts Commercially-available steel gage blocks were used as the stock blocks for the two mold inserts (0.25 inch rectangular Federal Grade II Gage Blocks Swiss Precision Instruments, Garden Grove, CA). These parts have an average surface roughness of approximately 1 nm [as determined by white light interferometry (Zygo NewView 3000, Zygo Corp. Middlefield, CT.)] and a flatness error less than roughly 51 nm, a value that is certified and NIST-traceable. The mold cavity which forms the microcantilever parts and the channel (the “sprue”) which delivers the polymer melt to the cavity are confined to the bottom and top inserts, respectively,

Fig. 4. Collapsed view of injection mold setup.

as shown in Fig. 3 (see Fig. 2 for a description of the microcantilevers and the base part). The top insert has a cylindrical hole (the sprue), which was machined on a conventional machining center (Benchman VMC-3000, Light Mafour flute chines Corp., Manchester, NH) using a 0.8 mm cobalt end mill at a plunge feed rate of 0.1 mm/min and a spindle speed of 5 kRPM. The bottom insert has a “large” cavity (in comparison to the microcantilevers) defining the support structure for the cantilevers, and this cavity was machined with the same setup just mentioned except that five plunge steps of 100 each were made and a linear feed rate of 1 mm/min was used. The cavities that define the microcantilever dimensions were then cut into bottom insert using a custom-made piezoelectric positioning stage-spindle setup (single plunge step, 0.05 mm/min plunge feed rate, 1 mm/min linear feed rate, 100 end mill, 50 kRPM spindle speed). The resulting mold and the cavity numbering convention are shown in Fig. 5. To avoid short shots, or incomplete mold filling resulting from premature melt freezing, two rectangular cross-sectioned heaters (120 V Sunrod Heaters, Sun Electric Heater Company, Danvers, MA) shown in Figs. 3 and 4 were used for mold insert heating, with the top two heaters wired in series, and likewise for the bottom. To reduce cycle times, a cooling system was employed to remove heat from the bottom insert by running water

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TABLE II PROCESSING CONDITIONS FOR THE VARIOUS POLYMERIC MATERIALS

Fig. 5. Optical micrograph of the mold used for this work and the “Cavity Number” scheme of Table I. TABLE I MOLD CAVITY GEOMETRIES

Fig. 6. Optical micrograph of a representative PS part with flash.

through the “Cooling Block” shown in Figs. 3 and 4 at flowrates of approximately 2 liters per minute. The white light interferometer described above was used to obtain topographical images of the mold. Using the interferometer software, line traces were placed in the topographic images at ten different positions (at equal intervals and perpendicular to the cavity length direction) along the cavity length direction for width and thickness measurements; line traces also were placed at five different positions (at equal intervals, perpendicular to the cavity width direction, and symmetric about the mid-width of the cavity) for length measurements. The mean values of these measurements are shown in Table I followed by the bracketed standard deviations. C. Material Selection Three polymers were used to produce microcantilever parts. The specific materials were chosen to show that injection molding can produce microcantilevers from amorphous polymers (Polystyrene, Chevron GPPS 3600, elastic modulus of 3 GPa), semicrystalline ( [54]) polymers (Polypropylene, Bassell ProFax 6323, elastic modulus of 1.45 GPa), and nanoclay-polymer composites (Nanoclay and Nylon Composite, Honywell-Aegis XA-2908 elastic modulus of 2.83 GPa). These materials will henceforth be called PS, PP, and NN6, respectively and were subjected to a repeatability study. The mold of Figs. 3 and 4 was mounted in a Sesame 0.080 Nanomolder (Medical Murray, Buffalo Grove, IL), which is an injection molding machine geared toward production of small-volume parts. The processing conditions used are listed in Table II.

Fig. 7. Optical micrograph of representative PP (top) and NN6 (bottom) microcantilevers.

III. REPEATABILITY STUDY RESULTS Parts were made from the various polymers and then manually removed from the mold (no mold release agent was necessary). Before discussion of the quantitative analysis, a qualitative discussion presents visual results of the parts produced. A. Qualitative Analysis The PS parts showed the most flash, as seen in Fig. 6; this amount of flash did not adversely affect the use of the PS parts as sensors. The flash thickness was determined via white light interferometry to be roughly 150–200 nm, indicating that true nanoscale injection molding may be possible if the mold cavities could be manufactured in a controlled fashion at the nanoscale, possibly via electrodischarge- or FIB-machining. The PP and NN6 parts exhibited minimal flash, and representative beams are shown in Fig. 7 for PP (top) and NN6 (bottom). It should be noted that during an initial study using molds different from, but produced with the same techniques outlined in this work, it was learned that as the molds see more use, the amount of flash


for a given polymer diminishes somewhat most likely due to the top mold insert repeatedly being pressed upon the bottom insert (with roughly 8 kN of force) which decreases, in an attritional fashion, any burrs that exist around the parting plane edges of the microcantilever cavities. Therefore, the flash exhibited by the PS could be due to youth of the mold, as PS was the first material used. Figs. 6 and 7 are taken from a plan view, so that the cantilever surface closest to the viewer is that which came into contact with the top mold insert (see Fig. 1)—this mold surface has an average roughness of approximately 5 nm while the mating top surfaces of the microcantilevers have an average roughness of approximately 10 nm (both determined via white light interferometry). Silicon parts measured with the same optical interferometer have a surface roughness of approximately 5–10 nm (depending on cantilever make and model), so the polymeric parts are rougher than some silicon parts, but further polishing of the mold top insert surface could reduce the surface roughness of the polymeric parts. For applications using the optical lever, an increased roughness could cause the laser spot to become excessively diffuse and reduce the sensor accuracy, although no such effect was observed for the cantilevers of this work. The swirl marks present in Figs. 6 and 7 are on the opposite side of the microcantilever, which is the side that came into contact with the bottom of the microcantilever channels that were machined into the mold bottom insert (see Fig. 1). At this thickness the microcantilever parts are nearly transparent, and hence the swirl marks, which are an artifact of the end-milling process used for channel machining, are visible. The white light interferometer-determined roughness of the machined channel bottom is roughly 200 nm. To reduce the roughness of this surface, alternative material removal methods may be useful. ECM, EDM, and FIB techniques have already proven feasible for microfabrication (ECM [19] and EDM [20]) and nanofabrication (FIB [21]). These methods could be capable of producing smaller, higher quality cantilever cavities with smoother surfaces. FIB could be used to reduce the surface roughness of the end-milled cavities of this work and FIB would likely be the best approach for making submicron thickness cantilever cavities—with material removal rates on the order of 10 [55], the machining time would be less - - nanocanthan roughly three minutes for a 25-5-0.25 tilever. The mold flash encountered in the PS parts of this work had a thickness of roughly 200 nm, indicating that IM at this thickness may be possible, although controlled cavity filling and warpage elimination would likely be challenging. It should be noted that making one surface of the microcantilever “rough” (for chemical bonding amelioration) and one side “smooth” (for laser reflection) could increase the efficacy of the microcantilever sensor as a whole [56]. B. Quantitative Analysis One goal of this work is to show that the new mold design could produce true MEMS parts in a repeatable fashion. One approach to quantifying repeatability and control status of the number of cantilever production process is to manufacture microcantilever parts and calculate statistical values such as the mean and standard deviation of measured parameters of the

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parts, and use these statistical values to gauge repeatability and process control. While this approach is valid, this work sought to produce a more rigorous repeatability judgement, and statistical process control (SPC) was turned to. The theory behind SPC is to take subsets of all the produced products, measure certain parameter(s) of interest, and then use this data to draw reasonable conclusions about the entire lot of parts produced, even though the desired parameter(s) of each part have not been measured. SPC allows conclusions to be drawn from many parts produced over a larger time frame without measuring each part, hence addressing repeatability in a larger sense than just measuring the first number of parts produced. The reader may review the literature for more information regarding SPC [57], [58]. The SPC repeatability analysis was employed, using the mold shown in Fig. 5, with the PS, PP, and NN6 materials. The scheme was to produce 230 parts from the mold from each material. A subgroup size of five with a total of ten subgroups was chosen; Western Electric Company Zone Rules (WECO rules) were employed to produce -bar charts. This is a total (ten subgroups per mold) (five parts per of (one mold) subgroup) (four microcantilevers per part) (one measurement per microcantilever per material) (three materials) equals 600 measurements. The subgroups consisted of parts – , for each cantilever 1–5, 26–30, 51–55, – made from each material. The parts were coated with 25 nm of gold (a standard procedure for microcantilever applications, with polymeric beams or otherwise) using an E-beam evaporator at a rate of 0.25 per second (CVC Products Electron Beam Evaporator, CVC Products Inc., Rochester, NY). The gold-coating allows sufficient laser energy to be reflected off the surface of the microcantilever for use in the AFM-type machine (Scentris System, Veeco Metrology, Santa Barbara, CA), used to measure the first bending mode resonant frequency of each microcantilever beam. The Scentris was operated in AC mode, which means that the cantilever parts were actuated piezoelectrically in a frequency sweeping fashion to locate the factor of all of first-mode resonant frequency. The quality the microcantilever systems (e.g., the different microcantilevers mounted in the Scentris system) was on the order of 100, which is commensurate with silicon type microcantilevers of similar resonant frequency in this specific piece of equipment in the authors experience. However, this is not the inherent thermally-induced quality factor of the beam itself (the commonly value in the literature) because the piezoelectric quoted actuation increases the system -factor—nonetheless previous work has shown that the inherent -factor of injection molded microcantilever beams is similar to that of silicon-type cantilevers for a variety of geometries [14]. The individual numerical results of the SPC analysis are detailed elsewhere [53], and none of the WECO rules were violated, indicating that the injection molding process was in control and repeatable. To test the validity of the SPC assumptions (e.g., independent, normally-distributed data and errors, constant variance of data from a distribution with a constant mean) autocorrelation, quantile-quantile, autoregressional conditional heteroscedasticity, and run-sequence analyzes were employed, and the resulting data satisfied the SPC assumptions.

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TABLE III MEAN AND STANDARD DEVIATION VALUES FOR THE SPC ANALYSIS

Fig. 8. Optical micrographs of deformation events of an injection molded PS microcantilever.

Table III shows the mean and standard deviation of the meaindividual first-mode bending resonant frequency surements from the SPC analysis for the different material and cavity geometry combinations. The data of Table III show – for the PS mean resonant frequency values ( – for the PP parts, and – for the parts, NN6 parts for the four different cantilever geometries) comparable to commercially-available silicon type cantilevers (e.g., – , and MikroMasch Veeco ESPW cited values: NSC12/Tipless/No Al “E” Type cited values: [33], [34]). Additionally, the standard deviation accounts for of the mean value for all material/geometry less than combinations, which, in conjunction with the SPC analysis, indicates that the molding apparatus discussed in this work can reproducibly fabricate polymeric microcantilever parts with mechanical behavior and production control comparable to existing, IC-produced microcantilever parts. C. General Notes The molds used in this work have produced well over one thousand parts from various polymers and, after the initial break-in period discussed, the parts showed no reduction in quality indicating that the molds are very durable and not as susceptible to fatigue-type failure as can be seen in certain ceramic molds (e.g., etched glass and silicon) when used in micromolding applications. To investigate the durability of injection molded cantilevers, an experiment subjecting PS microcantilever to severe deformation events was conducted. An injection molded PS cantilever was deformed three times in manners that are extreme when compared to the actual deformation the parts would see in use. Fig. 8 parts (a) through (h) show the events in chronological

order. Part (a) is the beam as it comes out of the injection mold, (b) is the first deformation event, and (c) is the recovered shape. Fig. 8(d) shows the second, more extreme deformation event, along with the recovered shape in (e). Fig. 8(f) shows the most extreme situation, which is essentially a 90 bending of the cantilever, with the recovered shape shown in (g). Finally, Fig. 8(h) shows the “flattened” shape, obtained by manually flexing the microcantilever of Fig. 8(g) in a downward fashion so that it is somewhat flat again. It should be noted that the configurations of Fig. 8(c), (e), and (h) are flat enough for use in an AFM using the optical lever detection technique. Similar results are seen for the other types of polymers employed (e.g., PP and NN6). That the microcantilevers are still feasible for use after this type of deformation implies that the plastic beams durable—on numerous occasions plastic beams were (accidentally) dropped on the floor and stepped on without damage, a feat that would likely destroy silicon-type beams. values of a microcanBy using the bulk elastic modulus tilever, the first-mode resonant frequency can be estimated as

(1)

where is the second moment of the cross sectional area of is the beam denthe beam, , is the beam length, and sity [59]. The results of the calculations are not presented for sake of brevity, but the trend for all materials at all geometries is that (1) systematically underestimates the resonant frequencies by roughly 60%. This is due to the frequency dependence for the polymer—the manufacturer-provided values are of obtained using uniaxial tensile tests, which are at a low strain rate compared to the beams oscillating at their . For polymers, the general behavior is an increase in the elastic modulus with increasing frequency [60], so the low estimate of (1) is not surprising as the elastic modulus of the beams at resonance is higher than the manufacturer-provided value. Indeed, vibration analysis is a tool to determine storage and loss moduli of viscoelastic materials [61].


Depending upon material, polymeric microcantilevers could be subject to time-dependent environmental effects changing their material properties or microstructure (e.g., long-term stability effects). For example, nylon polymers show a reduced elastic modulus upon sufficient exposure to water while photosensitive polymers can show radiation embrittlement (i.e., an increased elastic modulus upon exposure to certain types of electromagnetic radiation). These environmental effects could be deleterious or advantageous (e.g., a raised/lowered elastic modulus decreases/increases deflection sensitivity). The long-term stability of polymer microcantilevers could be an issue if the parts are left in certain environments for periods of time, however it is possible (if not probable) that a polymeric material could be used in the injection molding process which is resistant to said environment. Additionally, creep relaxation may become problematic if polymeric microcantilevers are kept at elevated temperatures (or kept at lower temperatures for extremely long time periods). It should be mentioned that silicon-type parts are not immune to all environments either (e.g., a silicon beam in a KOH environment), so there are situations where polymeric beams could be superior and situations where they could be inferior. IV. VAPOR-PHASE CHEMICAL SENSING To show that, in addition to being mechanical equivalents, polymeric microcantilevers also are functional equivalents to silicon-type parts for at least one application, plastic cantilevers fabricated via IM were used to monitor surface stress. The experiment chosen is somewhat of a benchmark for microcantilever sensors—the self-assembly of 6-alkanethiol monolayers on a gold surface. It was first studied with microcantilevers in 1996 and has been investigated by many others [62]–[66].1This experiment was chosen because (i) there is ample literature data for comparison and, more importantly, (ii) thiols can be used as DNA linkers hence enabling the use of polymeric microcantilevers for biosensing applications [67]. The thiols have a high affinity for the gold-coated surface of the microcantilevers and, after an initial contaminant desorption process, will bond to the gold and form a densely-packed, self-assembled monolayer (SAM) [68]–[70]. The thiols will protonate upon adsorption, and as the SAM forms, a surface layer of like charge will form, hence deflecting the microcantilevers away from the gold surface due to electrostatic repulsion. The general result from these works is that the surface stress induced on the gold coated surface is compressive at saturation (i.e., causes a deflection away from the gold surface), is proportional to thiol chain length, and has a magnitude on the order of 0.001–0.250 N/m, depending upon chain length and amount of surface coverage (i.e., if complete surface saturation is reached). A. Theory Previous work of Sader allows for calculation of the surface stress generated as a function of the beam’s material properties

HS(CH ) = 6.

1An n-alkanethiol has the chemical composition: a tail group- the alkanethiols used for this work had n

X where X is

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(obtained from the manufacturer), beam geometry, and trans. The gerverse deflection (i.e., in the thickness direction), mane result is given by (2) [71]

(2) ( is the position along the where cantilever length), ( is the position along the cantilever width), , where is the surface stress,2 is the cantilever bending (or flexural) rigidity, and are the beam material elastic modulus and Poisson’s ratio, , , and are the cantilever length, width, and thickness, and the are defined by (3). (3) By measuring the cantilever tip deflection (i.e., at , and ), (2) allows for calculation of the surface stress generated. It should be noted that Sader’s work (resulting in (2)) is an improvement upon the commonly-used Stoney’s equation [72] because it takes into account the boundary conditions at the fixed end of the cantilever. Additionally, it should be noted that the geometry of the microcantilever itself is assumed to be the mold cavity geometry; thermal shrinkage will play a role but, assuming a temperature change of 200 K and a coefficient of (both gross overestimates), thermal expansion of 100 , which is the surface stress change will be less than assumed negligible. 1) Deflection Sensitivity and the Advantage of Polymeric Microcantilevers: In terms of the cantilever sensitivity as a surface stress sensor (i.e., the change of deflection with a change in surface stress), polymeric materials can yield advantages over ceramic parts. From (2), the tip deflection sensitivity as measured with an AFM using the optical lever (one of the most common scales with the inmeasurement modes), verse of . Therefore, to maximize sensitivity the quantity should be minimized. Previous work has shown the minimum thickness achievable with the experimental setup of this [14]. The fact that the elastic modulus work to be roughly 2 2As is the convention in the literature, the term surface stress is used and its units are N/m, similar to a surface tension. What is meant is that t where is the average normal stress acting on a cross sectional area residing in a plane that is normal to the neutral axis of the beam-film composite (i.e., a “normal section”), and t is the film thickness. Hence is visualized as the normal force per unit width acting on a normal section of the film [72].

1 =

1

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Fig. 9. Deflection away from gold surface (top) and compressive surface stress generated (bottom) during monolayer self-assembly (from zero to 200 seconds the cantilever showed minuscule deflection) for a PS part exposed to a vapor-phase ethane-thiol.

of many polymers (e.g., PS, PP, and NN6) is times less than a silicon-type beam, implies that, given equivalent lengths, widths, and Poisson’s ratio, a polymeric beam can be made with times thicker (to keep the a thickness that is roughly equal for both polymeric and ceramic materials) quantity than a silicon type beam and yield the same displacement-sensitivity. Improving sensitivity by employing polymeric materials for a deflection-based sensor has been previously pointed out, as has the fact that thermal noise considerations could also favor the use of polymeric materials for microcantilever sensors [48], [53]. It should be noted that the Poisson’s ratio of many silicon-type materials will be less than that of most polymeric materials, but the microcantilever sensitivity is not as strong of a function in as it is of and . This discussion on sensitivity is not meant to imply that polymeric materials are always superior to ceramics, but for certain applications (namely surface stress sensing) they could very well be. It should be noted that the flatness error of the gage blocks used for this work is roughly 50 nm, and this limits the minimum injection-moldable thickness as do warping considerations (developed during the cooling stage of the injection molding cycle) so that the deflection sensitivity of polymeric materials fabricated via IM may not be able to match that of the new class of nanocantilevers [73]. Additionally, it should be noted that polymeric cantilevers can provide access to surface chemistry experiments that are not possible with silicon-type cantilevers (e.g., bonding antigens or antibodies directly to a PS microcantilever). B. Experimental Methods The experimental methods were modeled, as close as possible, after the work of Berger et al. [62]. To perform this DC mode (i.e., deflection-based) experiment, a vapor-borne ethanethiol was diffused into the Scentris system fluid flow cell (laboratory air-filled) where a gold-coated, four-beam, polymeric microcantilever part was located. The microcantilever deflection as

a function of time was recorded while the thiol diffused into the flow cell. The measured mold geometry values of Table III and the manufacturer-provided material property values, along with the surface stress-induced deflection modeling of Sader (i.e., (2) and (3)), allowed for calculation of the surface stress evolution as a function of time. C. Chemical Sensing Results As a representative result, the top of Fig. 9 shows the raw deflection data (for a PS part) while the bottom of Fig. 9 shows the calculated surface stress evolution. At steady-state, the four different microcantilevers in Fig. 9 show surface stress values of roughly 75 mN/m, 75 mN/m, 83 mN/m, and 77 mN/m for cantilevers made from cavity number 1, 2, 3, and 4, respectively. These values are in good agreement with each other and with obtained using the previously published reports of same experimental methods [62]. The four different cantilevers have slightly different geometries (see Table I), which is why the deflection curves in the top of Fig. 9 “collapse” into the surface stress curves in the bottom of Fig. 9. To gauge accuracy and repeatability of the monolayer formation-induced surface stress experiments, the thiols were flowed over ten cantilever parts (each with four microcantilevers) made from PS, PP, and NN6 (a total of 30 parts with a total of 120 cantilevers). Steady-state surface stress values were obtained from data analogous to that in Fig. 9 for each of the parts. Table IV shows the mean and standard deviation values obtained from these tests for the different materials. The values in Table IV (mean plus-or-minus one standard deviation sur, , and face stresses of for the PS, PP, and NN6 parts, respectively) show that the steady-state surface stress is in reasonable agreement (for the same thiol at similar concentration) with the literature [62], approximately 125 mN/m [66]) for values ( the PS, PP, and NN6 cantilever materials. These results demonstrate that injection-molded polymeric microcantilevers are fea-


MEAN

AND

TABLE IV STANDARD DEVIATION OF MEASURED SURFACE STRESS VALUES FROM DEFLECTION-BASED SENSING

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tion of a thiol SAM. The surface stress values obtained were commensurate with literature values obtained when using silicon-type microcantilevers for the same experiment. These results show that injection molding is now a feasible process for producing plastic MEMS microcantilever sensors as an economic alternative to more expensive and limited IC technologies. The mold design here could be reproduced inexpensively and allow more researchers to become involved in the field of micromolding and MEMS in general. ACKNOWLEDGMENT

sible deflection-based, vapor-phase, gold-thiol bonding sensors which produce experimental results in agreement with literature values obtained with the same experimental methods. D. Feasibility of IM Polymeric Parts in Other Applications? A logical question that arises is whether or not polymeric cantilevers can be used for applications other than gold-thiol surface stress sensing? The chemical sensing capability comes down to a question of surface chemistry due to the fact that the event inducing the surface stress occurs on one (or both) of the surfaces of the (possibly metal-coated) microcantilever. In many chemical and biochemistry sensing applications (e.g., gold-thiol bonding with varying thiols [62]–[66], DNA detection [67], prostate-specific antigen detection [35], and pH variation-induced surface stress [74] to name some), silicon-type beams have been used but are metal coated first, and then subsequent metal coating-analyte bonding manifests a surface stress. Since polymeric microcantilevers can be coated with the same metals as these silicon-type beams were, it could be possible to use IM polymeric microcantilevers in other chemical sensing applications—indeed previous work has shown that pH variation-induced surface stress measurements are possible using microcantilevers fabricated via IM [53]. It should be emphasized that the set of feasible experiments for IM polymeric microcantilevers and the set for silicon-type microcantilevers are not distinct nor are they equal—certain chemistries may only be possible with silicon surfaces and others may only be possible with polymeric surfaces. However, for many of the metal-coated microcantilever applications it is reasonable to assume that neither material class has a distinct or significant surface chemistry advantage (in terms of base the material and not the metal coating). It should be noted that silicon-type beams can and have been coated with some polymers but coating polymer beams with a silicon layer would likely be much more difficult [75], [76]. V. CONCLUSION This work details a novel injection mold design capable of producing plastic MEMS parts. Microcantilevers were chosen as the representative MEMS part and it was shown that the mold design was capable of producing these parts in a repeatable fashion from an amorphous polymer (PS), a semicrystalline polymer (PP), and a nanoclay-filled composite material (NN6). These injection-molded plastic parts are truly functional alternatives to their silicon-type counterparts as shown by their capability to monitor surface stress generation during the forma-

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Andrew W. McFarland received the B.S. degree in mechanical engineering from the University of California at Berkeley in May 2000, and the M.S.M.E. and Ph.D. degree s in mechanical engineering from the Georgia Institute of Technology, Atlanta, in December 2002 and December 2004, respectively. His research focused on microcantilever sensors, micromolding, and MEMS.

Jonathan S. Colton received the S.B.M.E., S.M.M.E., and Ph.D. degrees from the Massachusetts Institute of Technology, Cambridge. He is a Professor of Mechanical Engineering at the Georgia Institute of Technology, Atlanta. Dr. Colton is a Fellow of the American Society of Mechanical Engineers (ASME) and the Society of Plastics Engineers.