Self-aligned cantilever positioning for on-substrate measurements using DVD pickup head F.G. Bosco a, * , E.-T. Hwu b , S. Keller a , A. Greve a , A. Boisen a a Department of Micro- and Nanotechnology, Technical University of Denmark, Lyngby 2800, Denmark b Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan article info Article history: Received 14 September 2009 Received in revised form 11 December 2009 Accepted 15 December 2009 Available online 23 December 2009 Keywords: Cantilever-based sensing Microfabrication SU-8 DVD pickup head Chip holder abstract In this paper, we present a novel approach for measuring the resonant frequency of cantilevers fabricated in polymeric materials. We re-designed the use of a commercial DVD-ROM pickup head and combine it with a glass–polymer substrate in order to obtain a light and portable device to measure the resonant frequency of polymer cantilevers. The use of the Pyrex-SU-8 clamping substrate allows an easy replace- ment of the cantilever chips and a fast alignment process to the DVD-ROM laser beam. We show mea- surements of thermal noise for SU-8 and TOPAS cantilevers in air and liquid environment. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Cantilever-based sensors are promising miniaturized sensing tools for bio-chemical applications [1]. The mechanical response can be acquired through an optical setup, where a laser beam is fo- cused and collimated on the cantilever tip, and the reflected light is collected by a photodetector. These types of sensors can be used for bio-chemical detection when the cantilever is functionalized with a sensing layer that interacts with the target biomolecules. The sen- sitivity of the device is thus related both to the elasto-mechanical properties of the cantilever and to the opto-electronic characteristic of the readout setup. At present the optical equipment used to sense the change in the resonant frequency due to the selective binding of biomole- cules to the cantilever surface is typically big and bulky because precise and delicate laser alignment is required. In traditional ap- proaches a single chip is mounted on a holder, precisely aligned to the laser beam, and the cantilever’s resonant frequency is mea- sured through the oscillating position of the laser spot onto the photodetector. Typical alignment processes can take several min- utes of experienced human work. In this paper we present a novel approach to measure the change of resonant frequency of cantile- vers by using a light, compact and high throughput optical device, described in Section 2. The distinctive feature is the auto-align- ment of the laser beam to the cantilever tip, facilitating faster and more precise measurements. The optical readout of cantilever-based sensors has been re-de- signed and optimized combining the technology of commercial DVD-ROM readers [2] with a holding substrate. The holding sub- strate consist of a Pyrex support with an SU-8 structure on the top. 2. Setup In our system cantilever chips are clamped on a predefined SU- 8 structured holder while the DVD-ROM reader is placed 1 mm be- low the Pyrex substrate. Up to 100 chips can be clamped on the substrate. Once the sub- strate is mounted onto a XY-stage, simply placing the DVD pickup head 1 mm below the Pyrex surface will automatically align 800 cantilevers to the optical system, being each cantilever tip placed at the same Z distance from the optical head. This distinctive fea- ture saves several hours of alignment work when sequential mea- surements over several chips need to be performed, compared to existing optical setup. As a consequence, the holding substrate plays an important role in the setup. Chips need to be perfectly clamped to the substrate in order to keep precisely the position of the cantilevers in the XYZ space; furthermore the cantilevers need to be oriented parallel to the Pyrex surface. Fig. 1 shows schematically the layout of the experimental setup. Light (650 nm) emitted by a photodiode passes through a beam splitter and is collimated and focused by an optical system com- 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.064 * Corresponding author. E-mail address: filippo.bosco@nanotech.dtu.dk (F.G. Bosco). Microelectronic Engineering 87 (2010) 708–711 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee