Sensor fabrication chemical deposition

M.J. Schoning, Y.G. Mourzina, J. Schubert, W. Zander, A. Legin, Y.G. Vlasov and H. Luth, Can pulsed laser deposition serve as an advanced technique in fabricating chemical sensors , Sens. Actuators B Chem., 78(1-3)(2001) 273-278. 

Liu, Y. Koep, E. Liu, M., A highly sensitive and fast-responding Sn02 sensor fabricated by combustion chemical vapor deposition, Chem. Mater. 2005,17, 3997-4000... 

The sensitivity enhancement due to lasing action is a general principle that is independent of sensor architecture. Chemical sensitivity enhancements have been observed when the polymer is fabricated into a simple planar waveguide (Fig. 5.11a), deposited over a distributed feedback (DFB) grating (Fig. 5.11b) and coated on the exterior of an optical fiber (Fig. 5.11c). 

B. Min, S. Choi, Sn02 thin film gas sensor fabricated by ion beam deposition . Sensors and Actuators B Chemical, 98, 239-246, (2004). 

Other techniques have been used for the fabrication of thin-film metal-oxide gas sensors. At NIST in the USA, Cavicchi et al. (1995) and Semancik et al. (2001) produced gas sensors by chemical vapor deposition (CVD). By applying a current and thus heating the hotplate, sensing films could be deposited locally (i.e. only on heated active areas) using an adequate organ-ometaUic precursor. SnOj and ZnO films were obtained with tetramethyltin and diethylzinc in an oxygen atmosphere. They were deposited onto different seed layers, which played a significant role in terms of gas selectivity. 

We have approached the goal of functionalised monolayer fabrication by synthesis of a group of azobenzene-containing amphipathic molecules. Such molecules can be deposited by either chemisorption or physisorption and should allow hybrid films to be fabricated. The monolayers are reversibly photochromic (which might prove useful in sensor fabrication) and are capable of chemical activation for surface attachment of macromolecules, etc. In addition, the activated surface can be photochemi-cally modified so that photolithography could be used to fabricate surface structures such as danains of particular ccxnposition. With particular regard to conductive polyers, polyanilines could be locally formed as discussed later. 

A shadow-mask technique has been applied for the local metal deposition to exclude metal residues on other designs processed on the same wafer (Fig. 4.2b). Such metal residues may be caused by imperfections in the patterned resist due to topographical features on the processed CMOS wafers or dust particles. The metal film is only deposited in those areas on the wafer, where it is needed for electrode coverage on the microhotplates. This also renders the lift-off process easier since no closed metal film is formed on the wafer, so that the acetone has a large surface to attack the photoresist. Another advantage of the local metal lift-off process is its full compatibility with the fabrication sequence of chemical sensors based on other transducer principles [20]. 

MEMS (microelectromechanical systems) are systems with small device sizes of 1-100 pm. They are typically driven by electrical signals. To fabricate such systems materials like semiconductors, metals, and polymers are commonly used. MEMS technology fabrication is very cost-efficient. The structures are transferred by processes, which are applied to many systems on one substrate or even many of them simultaneously. The most important fabrication processes are physical vapor deposition (PVD), chemical vapor deposition (CVD), lithography, wet chemical etching, and dry etching. Typical examples for MEMS are pressure, acceleration, and gyro sensors [28,29], DLPs [30], ink jets [31], compasses [32], and also (bio)medical devices. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

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