Santeri Tuomikoski

Poly(dimethylsiloxane) electrospray devices fabricated with diamond-like carbon–poly(dimethylsiloxane) coated SU-8 masters

This study presents coupling of a poly(dimethylsiloxane) (PDMS) micro-chip with electrospray ionization-mass spectrometry (ESI-MS). Stable electrospray is generated directly from a PDMS micro-channel without pressure assistance. Hydrophobic PDMS aids the formation of a small Taylor cone in the ESI process and facilitates straightforward and low-cost batch production of the ESI-MS chips. PDMS chips were replicated with masters fabricated from SU-8 negative photoresist. A novel coating, an amorphous diamond-like carbon-poly(dimethylsiloxane) hybrid, deposited on the masters by the filtered pulsed plasma arc discharge technique, improved significantly the lifetime of the masters in PDMS replications. PDMS chip fabrication conditions were observed to affect the amount of background peaks in the MS spectra. With an optimized fabrication process (PDMS curing agent/silicone elastomer base ratio of 1/8 (w/w), curing at 70 degree C for 48 h) low background spectra were recorded for the analytes. The performance of PDMS devices was examined in the ESI-MS analysis of some pharmaceutical compounds and amino acids.

Characterization of SU-8 for electrokinetic microfluidic applications.

The characterization of SU-8 microchannels for electrokinetic microfluidic applications is reported. The electroosmotic (EO) mobility in SU-8 microchannels was determined with respect to pH and ionic strength by the current monitoring method. Extensive electroosmotic flow (EOF), equal to that for glass microchannels, was observed at pH > or =4. The highest EO mobility was detected at pH > or =7 and was of the order of 5.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer. At pH < or =3 the electroosmotic flow was shown to reverse towards the anode and to reach a magnitude of 1.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer (pH 2). Also the zeta-potential on the SU-8 surface was determined, employing lithographically defined SU-8 microparticles for which a similar pH dependence was observed. SU-8 microchannels were shown to perform repeateably from day to day and no aging effects were observed in long-term use.

Preparation of porous n-type silicon sample plates for desorption/ionization on silicon mass spectrometry (DIOS-MS)

This study focuses on porous silicon (pSi) fabrication methods and properties for desorption ionization on silicon mass spectrometry (DIOS-MS). PSi was prepared using electrochemical etching of n-type silicon in HF-ethanol solution. Porous areas were defined by a double-sided illumination arrangement: front-side porous areas were masked by a stencil mask, eliminating the need for standard photolithography, and backside illumination was used for the backside ohmic contact. Backside illumination improved the uniformity of the porosified areas. Porosification conditions, surface derivatizations and storage conditions were explored to optimize pSi area, pore size and pore depth. Chemical derivatization of the pSi surfaces improved the DIOS-MS performance providing better ionization efficiency and signal stability with lower laser energy. Droplet spreading and drying patterns on pSi were also examined. Pore sizes of 50-200 nm were found to be optimal for droplet evaporation and pore filling with the sample liquid, as measured by DIOS efficiency. With DIOS, significantly better detection sensitivity was obtained (e.g. 150 fmol for midazolam) than with desorption ionization from a standard MALDI steel plate without matrix addition (30 pmol for midazolam). Also the noise that disturbs the detection of low-molecular weight compounds at m/z < 500 with MALDI could be clearly reduced with DIOS. Low background MS spectra and good detection sensitivity at the 100-150 fmol level for pharmaceutical compounds were achieved with DIOS-MS.

Wafer-Level Bonding of MEMS Structures with SU-8 Epoxy Photoresist

An adhesive bonding method for fabrication of structures for microelectromechanical systems (MEMS) has been investigated. Negative photoresist SU-8 is used both as the structural material and as the adhesive material. Ultraviolet initiated crosslinking of SU-8 was investigated for bonding of small pillars as well as large uniform areas. Fabrication of thermally- and electrically insulated structures was also done and is reported here. Insulated structures are fabricated with SU-8 as roof, floor and sidewalls of the structure to ensure uniform wetting and surface electrochemistry for microfluidic applications.

Analytical characterization of microfabricated SU‐8 emitters for electrospray ionization mass spectrometry

We present a detailed optimization and characterization of the analytical performance of SU-8-based emitters for electrospray ionization mass spectrometry (ESI/MS). The improved SU-8 fabrication process presented here enhances patterning accuracy and reduces the time and cost of fabrication. All emitters are freestanding and enable sample delivery by both pressure-driven and spontaneous flows. The optimized emitter design incorporates a sharp, double-cantilevered tip implemented to the outlet of an SU-8 microchannel and provides highly sensitive ESI/MS detection. Moreover, the optimized design allows the use of relatively large microchannel dimensions (up to 200 x 50 microm(2), w x h) without sacrificing the detection sensitivity. This is advantageous with a view of preventing emitter clogging and enabling reproducible analysis. The measured limits of detection for the optimized emitter design were 1 nM for verapamil and 4 nM for Glu-fibrinopeptide B with good quantitative linearities between 1 nM and 10 microM (R(2) = 0.9998) for verapamil and between 4 nM and 3 microM (R(2) = 0.9992) for Glu-fibrinopeptide B. The measured tip-to-tip repeatability for signal intensity was 14% relative standard deviation (RSD) (n = 3; 5 microM verapamil) and run-to-run repeatability 4-11% RSD (n = 4; 5 microM verapamil) for all individual emitters tested. In addition, long-term stability of < 2% RSD was maintained for timescales of 30 min even under free flow conditions. SU-8 polymer was also shown to be chemically stable against most of the tested electrospray solvents.

Chapter Twenty Two – MEMS Lithography

Publisher Summary This chapter discusses the MEMS lithography in detail. Optical lithography is the mainstay of patterning in MEMS and its patterning is not driven by exposure wavelength reduction and related resolution improvement, but rather by process robustness, etch resistance, thick resists, double-side alignment, and the special needs for severely 3D structures. This chapter discusses lithography in four sections: issues to be answered before wafer processing begins; issues during lithography process; photoresist behavior in the process steps after lithography; special issues concerning thick photoresists. Before the first wafer is exposed, many decisions regarding the patterning process have been fixed. These include mask design, and the actual fabrication of physical mask plates that have to be consistent with the chosen photoresist. Resist polarity, negative or positive, and mirroring need to be specified in conjunction with mask order. The chapter briefly explains wafers in lithography. The process sequence of lithography is described. Prototypical positive resist process is also outlined. Different processes after lithography are explained. After the resist pattern is in place, wafers can experience several different process steps, which have their own specific requirements for resists and after wafer processing, resist removal is affected by the processes which the resist underwent. This chapter also explains the thick photoresist lithography.

Chapter 20 – MEMS Lithography

This chapter discusses the lithography considerations prior to wafer processing, including layout design such as the rules concerning minimum lines and spaces, required overlaps, recommended and/or forbidden shapes. The selection of the required photoresists for different applications is discussed. The process sequence of wafer lithography is described. The various applications of thick resists are covered briefly. As well as optical lithography with photomasks, other, more niche, patterning techniques are discussed.

Modified Porous Silicon Surfaces as DIOS-MS Sample Plates

This study presents a novel porous silicon (pSi) fabrication method for the matrix-free desorption/ionization on silicon mass spectrometry (DIOS-MS). The fabrication conditions and surface treatments of pSi on DIOS-MS performance are explored. Utilization of pSi sample plates in the analysis of low molecular weight pharmaceutical compounds is shown.

PDMS electrospray devices fabricated by PDMS-diamond-coated SU-8 masters

This study presents a new design for chip-based electrospray ionization mass spectrometry (ESI-MS), in which ESI is generated by direct spraying from a polydimethylsiloxane (PDMS) micro-channel electrokinetically without pressure assistance. PDMS devices were fabricated using SU-8-masters, which were coated with amorphous PDMS-diamond hybrid to increase their life-time. The presented technique allows rapid and low-cost batch-production of the ESI-MS chips.