Robert Andrew McGill

Electrostatically actuated resonant microcantilever beam in CMOS technology for the detection of chemical weapons

The design, fabrication, and testing of a resonant cantilever beam in complementary metal-oxide semiconductor (CMOS) technology is presented in this paper. The resonant cantilever beam is a gas-sensing device capable of monitoring hazardous vapors and gases at trace concentrations. The new design of the cantilever beam described here includes interdigitated fingers for electrostatic actuation and a piezoresistive Wheatstone bridge design to read out the deflection signal. The reference resistors of the Wheatstone bridge are fabricated on auxiliary beams that are immediately adjacent to the actuated device. The whole device is fabricated using a 0.6-/spl mu/m, three-metal, double-poly CMOS process, combined with subsequent micromachining steps. A custom polymer layer is applied to the surface of the microcantilever beam to enhance its sorptivity to a chemical nerve agent. Exposing the sensor with the nerve agent simulant dimethylmethylphosphonate (DMMP), provided a demonstrated detection at a concentration of 20 ppb or 0.1 mg/m/sup 3/. These initial promising results were attained with a relatively simple design, fabricated in standard CMOS, which could offer an inexpensive option for mass production of a miniature chemical detector, which contains on chip electronics integrated to the cantilever beam.

Micropreconcentrator for Enhanced Trace Detection of Explosives and Chemical Agents

The design, fabrication, and testing of a sorbent-coated microfabricated preconcentrator device in complementary metal-oxide-semiconductor is presented. As a sorbent-coated device, the preconcentrator is used to collect, concentrate, and deliver analyte sampled from air for analysis with a detector. The preconcentrator in this paper is based on a perforated flowthrough microhotplate structure that is coated with a sorbent layer to maximize vapor trapping efficiency. The coating sorbs the analytes of interest during the collection phase at ambient temperatures. A thermal desorption cycle is then used to rapidly heat the preconcentrator to 180 degC in 40 ms to release a concentrated wave of analyte. A finite-volume method was used to simulate the temperature distribution on a microhotplate and to model the time to reach the steady-state temperature. The experimental electrical measurements of the device were found to be in good agreement with the predicted values obtained using the finite-volume method. The preconcentrator device was demonstrated by interfacing to the front end of a handheld chemical agent detector and a handheld trace explosives detector. The preliminary results showed signal enhancement for the detection of the nerve agent simulant dimethylmethylphosphonate and the explosive 2,4,6-trinitrotoluene

Performance optimization of surface acoustic wave chemical sensors

Acoustic wave devices coated with a thin layer of chemoselective material provide highly sensitive chemical sensors for the detection and monitoring of vapors and gases. In this work, a variety of coating materials and coating deposition techniques have been evaluated on surface acoustic wave (SAW) devices. A novel thin film deposition technique, matrix assisted pulsed laser evaporation (MAPLE), is utilized to coat high quality polymer films on SAW devices, and conventional pulsed laser deposition is used to deposit a passivation layer of diamond-like-carbon on a SAW device surface to prevent water adsorption. In addition, chemoselective coatings are formed by covalent attachment of functionalized species to the silica surface of SAW devices. The self-assembled monolayer or near monolayer structures are designed to populate the SAW device surface with the desirable hexafluoroisopropanol moeity. The rapid kinetic signals achievable with the various coated SAW sensors during vapor tests are examined as a function of the coating material and the quality of the thin films. In parallel to the thin film deposition, growth, and vapor testing, the electrical characteristics of the SAW sensor have been characterized. The quality factor and residual phase noise of polymer coated SAW devices are examined, and a prediction of the theoretical limit of the phase noise performance of the loop oscillator is made.

Matrix Assisted Pulsed Laser Evaporation (Maple) of Polymeric Materials: Methodology and Mechanistic Studies

A new matrix assisted pulsed laser evaporation (MAPLE) technique has been developed at the Naval Research Laboratory, to deposit superior quality ultra thin, and uniform films for a range of highly functionalized polymeric materials. The MAPLE technique is carried out in a vacuum chamber, and involves directing a pulsed laser beam onto a frozen target, consisting of a polymer dissolved in a solvent matrix. The laser beam evaporates the surface layers of the target, where both solvent and polymer molecules are lifted into the evacuated gas phase. A solvent and polymer plume are generated incident to the substrate being coated. Si(111), and NaCl substrates coated with thin layers of polymer have been examined by a range of techniques including: optical microscopy, scanning electron microscopy and Fourier transform infra-red spectroscopy. Under optimum conditions the native polymer was transferred to the substrate without chemical modification as a highly uniform film. The MAPLE technique offers a number of advantages over conventional polymer deposition techniques, including the ability to precisely and accurately coat a relatively large or small targeted area with an ultrathin, and uniform coating with sub monolayer thickness control. Conventional pulsed laser ablation techniques can be utilized for coating a limited number of polymers, but we have found that for highly functionalized materials the native polymer structure is almost completely lost in the process. In contrast, when the MAPLE conditions are optimized the deposition of even highly functionalized polymeric materials proceeds with little effect on the intrinsic polymer structure.

Direct-write of sensor devices by a laser forward transfer technique

The use of direct-write techniques in the design and manufacture of sensor devices provides a flexible approach for next generation commercial and defense sensor applications. Using a laser forward transfer technique, we have demonstrated the ability to rapidly prototype temperature, biological and chemical sensor devices. This process, known as matrix assissted pulsed laser evaporation direct-write or MAPLE-DW is compatible with a broad class of materials ranging form metals and electronic ceramics to chemoselective polymers and biomaterials. Various types of miniature sensor designs have been fabricated incorporating different materials such as metals, polymers, biomaterials or composites as multilayers or discrete structures on a single substrate. The MAPLE-DW process is computer controlled which allows the sensor design to be easily modified and adapted to any specific application. To illustrate the potential of this technique, a functional chemical sensor system is demonstrated by fabricating all the passive and sensor components by MAPLE-DW on a polyimide substrate. Additional devices fabricated by MAPLE DW including biosensors and temperature sensors and their performance are shown to illustrate the breadth of MAPLE DW and how this technique may influence current and future sensor applications.

Direct writing of electronic materials using a new laser-assisted transfer/annealing technique

MAPLE direct write is anew laser-based direct write technique which combines the basic approach employed in laser induced forward transfer with the unique advantages of matrix assisted pulsed laser evaporation. The technique utilizes a laser transparent donor substrate with one side coated with a matrix consisting of the electronic material to be transferred mixed with an organic binder or vehicle. As with LIFT, the laser is focused through the transparent substrate onto the matrix coating. When a laser pulse strikes the coating, the matrix is transferred to an acceptor substrate placed parallel to the donor surface. Ex situ thermal or laser treatments can be used to decompose the matrix and anneal the transferred material, thus forming structures with the desired electronic properties. MAPLE DW is a maskless deposition process designed to operate in air and at room temperature that allows for the generation of complex patterns with micron scale linewidths. The various structures produced by MAPLE DW were characterized using 3D surface profilometry, scanning electron microscopy and optical microscopy. The electrical resistivity of the silver metal lines made by MAPLE DW was measured using an impedance analyzer. Patterns with Zn2SiO4:Mn powders were fabricated over the surface of a dragon fly wing without damaging it. An overview of the key elements of the MAPLE DW process including our current understanding of the material transfer mechanisms and its potential as a rapid prototyping technique will be discussed.

Matrix-assisted pulsed-laser evaporation (MAPLE) of functionalized polymers: applications with chemical sensors

A novel polymer processing technique, matrix assisted pulsed laser evaporation (MAPLE), for the deposition of organic and inorganic polymers and other materials, as ultrathin and uniform coatings has been developed. The technique involves directing a pulsed excimer laser beam onto a frozen matrix target composed of the polymeric material in a solvent. The process gently lifts polymeric material into the gas phase with no apparent decomposition. A plume of material is developed normal to the target, and a substrate positioned incident to this plume is coated with the polymer. The MAPLE technique offers a number of features that are difficult to achieve with other polymer coating techniques, including: nano-meter to micron thickness range, sub monolayer thickness precision, high uniformity, applicability to photosensitive materials, and patterning of surfaces. Highly functionalized polysiloxanes have been synthesized and deposited on a range of substrates by the MAPLE technique and characterized by: infrared spectroscopy, and optical microscopy. High quality, uniform and adherent polysiloxane coatings are produced by the optimized MAPLE technique. The physicochemical properties of the coating are unaffected by the process, and precise thickness control of the coating is straightforward.

Laser Direct Writing of circuit elements and sensors

A novel approach for maskless deposition of numerous materials has been developed at the Naval Research Laboratory. This technique evolved from the combination of laser induced forward transfer and Matrix Assisted Pulsed Laser Evaporation (MAPLE), and utilizes a computer controlled laser micromachining system. The resulting process is called MAPLE-DW for MAPLE Direct Write. MAPLE-DW can be used for the rapid fabrication of circuits and their components without the use of masks. Using MAPLE-DW, a wide variety of materials have been transferred over different types of substrates such as glass, alumina, plastics, and various types of circuit boards. Materials such as metals, dielectrics, ferrites, polymers and composites have been successfully deposited without any loss in functionality. Using a computer controlled stage, the above mentioned materials were deposited at room temperature over various substrates independent of their stage, the above mentioned materials were deposited at room temperature over various substrates independent of their surface morphology, with sub-10micrometers resolution. In addition, multilayer structures comprising of different types of materials were demonstrated by this technique. These multilayer structures from the basis of prototype thin film electronics devices such as resistors, capacitors, cross-over lines, inductors, etc. An overview of the result obtained using MAPLE-DW as well as examples of several devices made using this technique is presented.

Sorbent coatings for detection of explosives vapor: applications with chemical sensors

A series of chemoselective polymers have been designed and synthesized in order to enhance the nitroaromatic sorption properties of coated acoustic wave devices. Acoustic wave devices coated with a thin layer of chemiselective polymer can provide highly sensitive transducers for the detection of vapors or gases. The sensitivity and selectivity of the sensor depends on several factors including the chemoselective coating used, the physical properties of the vapor(s) of interest, the selected transducer, and the operating conditions. To evalute the effectiveness of the chemoselective coatings a polynitroaromatic vapor test bed was utilized to challenge polymer coated SAW devices. Detection limits with the coated SAW sensors, as tested under laboratory conditions, are determined to be in the lower parts per trillion range. FTIR studies were undertaken to determine the nature of the polymer-polynitroaromatic interactions.

Recent developments in sorbent coatings and chemical detectors at the Naval Research Laboratory for explosives and chemical agents

New chemiselective polymers have been developed to enhance the nitroaromatic sorption properties of coated acoustic wave (AW) devices. The sensitivity and selectivity of polymer-based sensors depends on several factors including the chemiselective coating used, the physical properties of the vapor(s) of interest, the selected transducer, and the operating conditions. Detection limits with the coated SAW sensors, tested under laboratory conditions, are determined to be < 100 parts per trillion for 2,4-dinitrotoluene. A new SAW based chemical vapor detector the NRL p-CAD has been developed with dramatically improved signal kinetics offering T95 response times of less than 0.1 second for a wide range of organic compounds including the nerve agent simulant and agent precursor material dimethylmethylphosphonate. In addition, the NRL p-CAD system offers a rapid 2s baseline reset virtually eliminating baseline drift issues associated with changes in temperature and relative humidity. The p-CAD system has been successfully tested in both ground and unmanned aerial vehicle testing.