111Equation Chapter 1 Section 1Proof of concept coded aperture miniature mass spectrometer using a cycloidal sector

mass analyzer, a carbon nanotube (CNT) field emission electron ionization source, and an array detector

Supplementary Material

Jason J. Amsden,†,$ Philip J. Herr,†,$ David M. W. Landry,† William Kim,† Raul Vyas,† Charles B. Parker,† Matthew P. Kirley,†

Adam D. Keil,@ Kristin H. Gilchrist,‡ Erich J. Radauscher,† Stephen D. Hall,‡ James B. Carlson,‡ Nicholas Baldasaro,‡ David

Stokes,‡ Shane T. Di Dona,† Zachary E. Russell,†,& Sonia Grego,‡ Steven J. Edwards,* Roger P. Sperline,* M. Bonner Denton,*

Brian R. Stoner,‡ ,† Michael E. Gehm,† and Jeffrey T. Glass†,^

Author Affiliations:

†Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA 27708

‡Engineering and Applied Physics Division, RTI International, Research Triangle Park, NC USA 27709

@Broadway Analytical, LLC Monmouth, IL USA 61462

*Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ USA 85721

&Present Address: Ion Innovations, Roswell, GA USA 30075

$These authors contributed equally

^Corresponding Author:

Jeffrey T. Glass

Department of Electrical and Computer Engineering

Duke University

Durham, NC 27708

(919) 660-5431

jeff.glass@duke.edu

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1 Supplemental Material

1.1 C-CAMMS system

Figure S 1. C-CAMMS subsystems. (a) Block diagram showing the C-CAMMS subsystems. (b) Photograph of the C-CAMMS prototype with various components labeled.

1.2 Ion source design and fabrication

Nier-type ion sources ionize gasses with an electron beam perpendicular to the ion’s trajectory. They comprise an electron

source, ionization volume, collimating magnetic field, and extraction optics (Figure S 2). The magnetic field and electron beam

are aligned to allow high electron transmission through the ionization volume with minimal perturbation of the ion beam [1].

This structure is usually constructed using metal plates and a housing to apply the required potentials. Potentials are applied to

the ionization volume and ions are directed out of the ionization volume through the extraction aperture and then accelerated

through a spatial filter with minimal energy and angular dispersion. To fabricate a miniature Nier-type ion source, we employ a

silicon wafer with an etched cavity where we grow CNT field emitters at the bottom for an electron source. The ionization

volume is constructed using an electroformed metal box with grids to allow transmission of electrons and analyte into the

ionization volume from the CNT electron source and sample inlet, respectively. An LTCC scaffold provides a structure that

encapsulates the electron source and ionization volume as well as positioning the extraction aperture and spatial filter above

the ionization volume. The LTCC also contains electrical vias and traces to apply potentials to the electron source, ionization

volume, and extraction optics. A detailed description of the design and fabrication of this miniature ion source follows.

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Figure S 2. Cross section of a Neir type ion source including cathode, ionization volume, collimating magnetic field, extraction optics. Dimensions are given in blue for the extraction aperture and spatial filter. Not to scale.

1.2.1 Microfabricated carbon nanotube cathode design and fabrication

The electron source used in this ion source is a 2.5 mm x 1.2 mm heavily doped n-type silicon chip 550 µm thick with an

etched cavity 20 µm deep coated with CNTs. Prior to cavity formation, the wafer was partially diced so that the exposed

substrate edges would be insulated in a later step. Photolithography was used to define 2 mm x 0.5 mm cavities that were

etched into the silicon substrate to a depth of 20 µm using deep reactive-ion etching (DRIE) (Figure S 3a). Following DRIE, a

1 µm oxide layer was deposited over the entire wafer using plasma-enhanced chemical vapor deposition (Figure S 3b). This

oxide electrically isolates the silicon substrate from the metal electron inlet that is integrated into the ionization volume.

Photolithography with a dry film resist (Figure S 3c) was used to selectively etch the oxide from the bottom of the cavities

using reactive ion etching. A 5 nm thin film of iron was electron beam evaporated onto the exposed silicon in the cavity

(Figure S 3d). This iron serves as the catalyst for CNT growth. The dry film mask was removed and the silicon wafer was

cleaved into 1.2 mm x 2.5 mm pieces along the previously diced lines (Figure S 3e). Finally, 15 µm tall, vertically aligned

carbon nanotubes were grown in the cavity using a microwave plasma enhanced chemical vapor deposition system [2] (Figure

S 3f).

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Figure S 3. Schematic of silicon wafer processing for the electron source; (a) silicon wafer after partial dicing and DRIE, with a 20 µm deep cavity and dicing marks; (b) 1µm PECVD oxide is grown on wafer; (c) the wafer is coated with a patterned dry film resist; (d) DRIE of cavity oxide followed by deposition of Fe catalyst; (e) dicing of silicon wafer; (f) after CNT growth; (g) SEM image of CNT bundle after MPCVD growth; (h) optical image of ionization volume before assembly; (i) SEM image of ionization volume after assembly.

1.2.2 Ionization volume design and fabrication

For the ionization volume, we use a single piece of metal fabricated using a double layer electroforming process (Thin Metal

Parts, Inc., Colorado Springs, CO). Parts are manufactured as a flat, 50 µm thick metal structure resembling an unfolded box

(Figure S 3h). Score marks aid in folding the metal into the rectangular ionization volume. The shorter side panels include a

mesh that serves as the electron beam inlet. The mesh sides of our ionization volume are placed in contact with the CNT

cathodes discussed in the previous section and adhered together using a nonconductive epoxy. A potential placed between the

cathode and ionization box creates the conditions necessary for field emission. The ionization volume and adhered cathode can

be seen in Figure S 3i. The dimensions of our ionization box are 4.55 mm x 2.65 mm x 1.4 mm. This geometry was chosen for

reasons that will be discussed in the ion source simulation section.

1.2.3 LTCC design and fabrication

Like the MEMS device with integrated CNT cathodes described previously, the ion source uses LTCC as a scaffold to position

and provide electrical connection to the electron source, ionization volume, extraction optics, and sample inlet. The LTCC

starts as 0.1 mm thick flexible layers, called tape, made of ceramic particles in an organic matrix. The layers of tape are cut to

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size with cavities, vias are punched and filled, and electrodes are screen printed. Some layers are laminated together at high

pressure before firing at 850 C. Firing removes the organic material and densifies the laminated layers. For this ion source,

Omega Micro Tech, Inc fabricated the LTCC parts from DuPont™ 951 GreenTape™. The ion source has 30 individual layers

laminated into 3 separate stacks. Figure S 4a and Figure S 4b shows the top and bottom exploded view of the three stacks of

LTCC respectively.

In the bottom stack, cavities form an inlet for the gas analyte to reach the ionization volume and an outlet for neutral gas to

exhaust from the ion source. A larger cavity provides a space for the CNT cathodes and the ionization volume. At the bottom

of the large cavity are gold pads to electrically connect the cathode and ionization volume to a power supply. Each layer also

includes gold traces and vias to supply the middle and top stacks with voltage to the extraction aperture. Additionally, a

grounded gold lattice shields the ionization volume from external electric fields and shields the internal electric field from

perturbing the uniform electric field needed for the cycloidal mass analyzer.

The middle stack provides the electrical connection and mechanical structure to hold the extraction aperture. The cavity in this

stack provides space for the ionization volume and prevents short circuits from the grounded lattice. The cavity on top provides

space for the extraction aperture. Electrical connection to this aperture is provided by gold pads located at the bottom of this

cavity. The extraction aperture is a single square hole with dimensions of 1 mm x 1 mm. The aperture is a 75 µm-thick

stainless-steel sheet fabricated using laser machining (Thin Metal Parts, Inc., Colorado Springs, CO). The top stack has a

cavity cut through all 10 layers to allow ions to pass through the spatial filter, out of the ion source, and into the mass analyzer.

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Figure S 4 (a) CAD model exploded view from top of LTCC scaffold. (b) CAD model exploded view from bottom of LTCC scaffold. (c) Photograph of ionization volume inserted into bottom stack. (d) Photograph of assembled LTCC ion source. Penny for scale.

1.2.4 Ion source assembly

The ion source assembly starts with bending the unfolded ionization box (Figure S 3h) and adhering the cathode to the sides

(Figure S 3i). The assembly is electrically connected to the gold pad at the bottom of the large cavity in the first multilayer

LTCC stack using conductive silver epoxy. A photograph of the ionization volume inserted into the bottom stack is shown in

Figure S 4c. The extraction aperture is attached to the middle stack. The bottom and middle stacks are electrically connected

using conductive silver grease and adhered using non-conductive Loctite 380 epoxy. Using a custom metal fixture, the middle

stack is aligned with and placed on top of the bottom stack. The spatial filter is attached to the top stack and joined with the

middle stack using the same method. The dimensions of a fully assembled ion source are 23 mm x 7 mm x 3 mm. Figure S 4d

shows a photograph of an assembled ion source.

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1.2.5 Ion source simulation

Figure S 5. Results from ion source SIMION simulation using the experimentally optimized voltages on the ionization volume, extraction aperture, and spatial filter. Ion trajectories are colored red, electron trajectories are black, and 5 V spaced equipotential lines are blue. (a) 2D cross section in the ZY-plane passing through the center of the ionization box. (b) 2D cross section in the XY-plane passing through the center of the ionization box.

1.3 C-CAMMS results

Figure S 6. Comparison of a spectrum of a mixture of lab air and methanol taken using C-CAMMS and a Stanford Research Systems RGA-300 using electron ionization with 70 eV electrons. Also included is simulated data of methanol from the NIST Chemistry Webbook. Here, a reservoir of liquid methanol was placed before the needle valve. When the needle valve is opened, methanol vapor as well as lab air is introduced into the mass spectrometer. Spectra are normalized to the peak at m/z = 31. The spectrum contains a mixture of methanol (m/z = 15, 28, 29, 30, 31, 32, 33), water (m/z = 17, 18), oxygen (m/z = 32, 16), and nitrogen (m/z = 28, 14). The spectra using C-CAMMS and the RGA-300 both show different relative intensities for oxygen, nitrogen, water, and methanol. This is due to the different electron energy and energy spread used for

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ionization. The RGA-300 uses ~70 eV electrons, while C-CAMMS uses ~200 eV electrons.

1.4 References

1. Vestal, M.L.: Methods of Ion Generation. Chemical Reviews 101, 361-376 (2001)

2. Cui, H., Zhou, O., Stoner, B.R.: Deposition of aligned bamboo-like carbon nanotubes via microwave plasma enhanced

chemical vapor deposition. Journal of Applied Physics 88, 6072-6074 (2000)

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