Microelectronic Engineering 84 (2007) 1190–1193 www.elsevier.com/locate/mee
High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications Ling Wang a,*, Robert Stevens a, Adnan Malik a, Peter Rockett b, Mark Paine c, Paul Adkin a, Scott Martyn b, Katherine Smith c, John Stark c, Peter Dobson b a Rutherford Appleton Laboratory, CCLRC, Didcot OX11 0QX, UK Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK Department of Engineering, Queen Mary University of London, London E1 4N5, UK b
c
Available online 2 February 2007
Abstract With the trend in chemical and biological analysis for a reduction in sample size, the demand is for smaller and more finely controlled droplet emission from electrospray micronozzles. One of the key issues is to make reliable electrospray emitters with smaller diameters and high aspect ratio (length to diameter) to ensure a high value of electric field at the nozzle tip. Silica nozzles with aspect-ratio of over 20 were designed and fabricated in both single and array formats. An investigation of silicon deep etch by DRIE was performed and achieved a depth greater than 200 lm for 10 lm diameter channels. The surface wettability was also studied and the result showed that a thin gold film with low surface energy would offer the desired level of hydrophobic performance and be sufficiently robust for most micronozzle applications. The microfabricated single and linear-array nozzles were tested in the operation modes of on-line pumped and off-line isolated. The on-line tests for the single nozzles resulted in stable cone-jet electrospray with flow rates as low as 100 nl/ min at voltage of a few kV. The off-line tests for the linear-array nozzles showed cone-jet pulsed electrospray with the production of picolitre droplets. Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. Keywords: Micronozzles; DRIE; Elecrospray; MEMS
1. Introduction A rapid growing interest on miniaturized analytical instrumentation has emphasized the need to develop microdevices capable of delivering and manipulating nanoliter quantities of chemical and biological samples. Electrospray has become one of the most versatile techniques for miniaturized analytical systems such as mass spectrometry, and nanoparticle deposition tools because of its ability to produce fine and controllable droplets [1,2]. A key component in the electrospray system is a micro-nozzle which takes the form of a fine capillary with a terminating nozzle tip usu-
*
Corresponding author. Tel.: +44 1235 445829; fax: +44 1235 446283. E-mail address:
[email protected] (L. Wang).
ally a few microns in dimension. In order to obtain smaller droplets emitted from the nozzle, the dimension of the nozzle has to be scaled down [3]. To maintain a stable electrospray mode, a high value of electric field is required at the nozzle tip. The results from finite element modeling has shown that to achieve the desired value, the nozzle aspect ratio should be greater than 20 [4]. Historically, a conventional nozzle is made from glass tubing, called a capillary nozzle [5], which is easy to make but limited to a single nozzle assembly and difficult to integrate with other microfluidic components on a single chip. Recently, MEMS (microelectromechanical systems) technology has been widely accepted as an innovative tool for fabrication of microdevices and sensors on glass or silicon substrate. Advantages of MEMS fabrication for micronozzles include flexibility in the design process relat-
0167-9317/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.116
L. Wang et al. / Microelectronic Engineering 84 (2007) 1190–1193
ing to nozzle shape and ease of fixture for multiple nozzle assembly. Other benefits are lower manufacturing cost and greater sub-system integration. There are several MEMS technologies that can be used for making highaspect-ratio microdevices which include LIGA [6], hot embossing [7], precision injection molding [8] and bulk micromachining [9]. Bulk micromachining is one of the most popular MEMS techniques to realize micro-structures using standard UV-light lithography and plasma deep etch on silicon wafers. It is a well-developed process for making quick prototype devices and compatible with the processes used for fabricating integrate circuits. Using this technique, it is possible to build additional parts onto a single chip, such as a micro-pump and filters. The various processes involving the fabrication of a high-aspect-ratio silica nozzle are reported in this paper. In particular, this includes details of nozzle plate fabrication, capillary attachment to the nozzle plate, rigidity test, and surface modification studies. Also, electrospray test results from single and linear-array nozzles are reported.
2. Fabrication The micro-nozzles are fabricated on a 700 lm thick silicon wafer with designs of single nozzle, cluster nozzle and linear-array nozzles. The fabrication is a three masks process, mainly using DRIE (Deep React Ion Etch) for silicon etching and AZ9260 photoresist for lithography. The fabrication begins with a thin LPCVD silicon nitride deposited on the both sides of the wafer and a 2.5 lm PECVD SiO2 on the backside of the wafer as shown in Fig. 1a. After the SiO2 and silicon nitride is patterned, the backside of the wafer is lithographed again for silicon etching (Fig. 1b). Next, the photoresist is removed and further silicon etching is carried out using the SiO2 and SiN as a mask, as shown in Fig. 1c. With 1 lm PECVD SiO2 deposited on the backside of the wafer as a protect layer, the 10 lm diameter nozzle channels is etched 200 lm deep from the front side of the wafer using thick photoresist as a mask (Fig. 1d). The backside of oxide/ nitride layer is then removed, followed by a thermal oxidation in furnace to form the 3 lm thick of silica nozzle walls, presented in Fig. 1e. The next step, shown in Fig. 1f, is to etch away the surrounding silicon to reveal the silica nozzles with a backing wafer on. The fabrication of the nozzle plates is complete at this stage, and the nozzle plates can be picked up individually from the backing wafer and assembled with a glass capillary, as shown in Fig. 1g. The critical step is the anisotropic silicon etching of the 10 lm diameter channels. An investigation of the silicon deep etching showed that the silicon etch rate was not linear with the etch time. This indicated that the smaller and the deeper the channel, the lower the etch rate. For example, the silicon etch rate is decreased by 40% when the
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a b c d e f g Fig. 1. Fabrication process sequence.
depth of the channels is over 150 lm. A SEM image of silicon deep channels is shown in Fig. 2 with 20 lm diameter and 240 lm depth. Fig. 3 shows the fabricated nozzles distributed on silicon substrate after the silicon blanket etch. The large arrays of columns are designed to increase the loading effect for small channel bore silicon etching. The micronozzle assembly is to attach the fabricated nozzle plate to a glass capillary which provides a passage for the liquid supply to the electrospray emission. A special kit is made for the assembly work with a micro-movement on x, y and z directions for alignment of the glass capillary onto the backside of the nozzle plate. The two parts are glued together with UV curable epoxy. Fig. 4 shows an example of the assembled structure of a linear-array nozzle. The integrity of the nozzle assembly was determined by
Fig. 2. SEM cross-section of silicon deep etched channels with diameter of 20 lm.
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SiO2 and SiN films have higher surface energies (contact angles 70°). Gold film and conducting DLC (diamond-like-carbon) film have been chosen for the nozzles surface coatings because of their lower surface energies, good conductivity and chemical and biological compatibility. 3. Test results Fig. 3. Micromachined silica nozzles on silicon substrate before separated to nozzle plates. The insets present a single nozzle plate and a linear-array nozzle plate released from the substrate.
Nozzle plate
Capillar
1mm
Fig. 4. A linear-array nozzle plate is assembled to a capillary. The insets are the images of the backside of the plate and the nozzle tips.
tensile testing. The results indicated that a maximum force of >100 kg/cm2 (>1400 psi) could be supported by a single nozzle. The blockage of the nozzle tip was also examined using DI water spray through the nozzles, as shown in Fig. 5. For the stability of electrospray operation, it is important to prevent wetting on the outer surface of the nozzles. This can be achieved by coating the outer surface with low surface energy thin film materials. The surface energies on different materials were investigated by water droplet contact angles measurements. The results confirmed that Si,
The fabricated micro-nozzles were extensively tested using electrospray systems developed at Oxford University and Queen-Mary College University of London. In the case of the Oxford facility, the electrospray tests were carried out in a sealed chamber supplied with a controlled bleed of bottled gas. The assembled nozzle was mounted vertically at the top of the chamber and supplied with liquid from a precision controlled nanofluidic pump, as shown in Fig. 6a. Electrospray operates as an atmospheric pressure ionization process which occurs when an electric field distorts the capillary meniscus liquid/gas interface into a prolate spheroid and subsequently induces an emission point as a liquid jet. Under the desired running conditions, the jet will stabilize into a cone-jet identified as the Taylor cone [10]. The electrospray images in Fig. 6b show that the cone-jet size and shape changes with the applied voltage from 2.5 kV to 2.7 kV with flow rate of 100 nl/min. The variation in curvature of the meniscus is an indication of the tensile forces that are required to maintain equilibrium between electromechanical and hydrodynamic forces to preserve the cone-jet mode of electrospray operation. Electrospray has been used for metal nanoparticle and active biomolecule deposition for nanotechnology and biosensors applications. An important factor in the electrospray deposition is the level of particles throughput. A practical way to increase the flux is to use a cluster or an array of nozzles to produce multiple electrospray simultaneously. The microfabricated linear-array nozzle reported here has a line of 10 nozzles with 10 lm diameter, 250 lm length and 200 lm spacing. An initial test on microarray deposition using the linear-array nozzle in offline mode is presented in Fig. 7. It was observed that electrospray emission occurred simultaneously from the 10 nozzles at a voltage of 1.1 kV for a 30 ms pulsed operation.
a
b
2.50kV 9.04nA 2.60kV 9.00nA 2.70kV 7.54nA
Fig. 5. A blockage test on a linear-array nozzle with DI water spray through the nozzle tips.
Fig. 6. (a) Electrospray test chamber and (b) electrospray from a single nozzle at different voltages (50% ethanol/50% water, 100 nl/min).
L. Wang et al. / Microelectronic Engineering 84 (2007) 1190–1193 200μm
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nozzles showed a pulsed electrospray with delivery of picolitre droplets. The implications that arise from the reported work are that MEMS fabricated devices have an important role to play in future developments in electrospray technology.
200μm
Acknowledgments This work is supported by the Research Council UK’s Basic Technology Grant GR/R8703/03. Fig. 7. Image of the linear-array nozzle (part of it) during on–off pulsed electrospray to make an array of microparticles on silicon substrate.
A microarray with droplet of picolitre is deposited on a silicon substrate which is placed 200 lm from the nozzle tips. 4. Summary Silica nozzles with 10 lm diameter and 250 lm long are fabricated using bulk micromachining technology with single and array designs. The fabrication process for the highaspect-ratio nozzles has been optimized from wafer fabrication to nozzle plate assembly. The silica nozzle can stand >1400 psi pressure on a single nozzle and the surface energy is reduced by gold thin film coating. The fabricated silica nozzles have been successfully tested for electrospray. The on-line tests for the single nozzles resulted in stable cone-jet electrospray with flow rates as low as 100 nl/min at voltage of a few kV. The off-line tests for the linear-array
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