Silicon lapping

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

The first LAPS utilized silicone nitride (S3N4) as a pH-sensitive layer [68], A light-addressable high resolution pH imaging sensor was applied to the detection of spatially resolved metabolic activity of Escherichia coli colonies on agar medium [69], For a silicone substrate thickness of 20 pm the reported spatial resolution was about 10 pm. The observed pH distribution was in good agreement with the results of simulation based on a two-dimensional diffusion model. 

H. A. Lawler, R. W. Ohnsorg, R. S. Storm, and D. A. White, "Progress on Al lAP Silicon Carbide Component Development," Proceedings of the Annual Automotive Technology Development Contractor s Coordination Meeting Dearborn, Mich., Oct. 1990. 

The benefit of such a LAPS sandwich structure consists in a easy manufacturing process, where no patterning and masking as well as encapsulation of conducting tracks are needed (see Fig. 5.1). In the measurement set-up, the LAPS structure itself will be embedded in a measurement chamber. The flat uniform surface of the LAPS assists a proper sealing by, e.g., an O-ring, silicone or rubber material. Finally, only the sensitive membrane will stay directly in contact with the analyte under test. 

Due to the inability to deposit eleetrochemically stable gate-insulating materials for GaAs, another approach was developed based on amorphous silicon (a-Si), prepared as a thin layer for LAPS devices on transparent glass substrates [40]. The diffusion length in this material was reported to be as small as 120 nm [41] and a resolution down to 1 gm has been demonstrated, which was mainly limited by the optical set-up. The electrochemical properties of the a-Si-based structures were investigated later with a LAPS device thus, the above results for SPIM were transferred back and proved for the LAPS, too [42,43]. 

W. Moritz, T. Yoshinobu, F. Finger, S. Krause, M. Martin-Fernandez and M.J. Schoning, High resolution LAPS using amorphous silicon as the semiconductor material, Sens. Actuators B Chem., 103 (2004) 436—441. J.C. van den Heuvel, R.C. van Oort and M.J. Geerts, Diffusion length measurements of thin amorphous silicon layers, Solid State Commun., 69(8) (1989) 807-810. 

T. Yoshinobu, M.J. Schoning, F. Finger, W. Moritz and H. Iwasaki, Fabrication of thin-film LAPS with amorphous silicon, Sensors, 4 (2004) 163-169. 

The substrate may be partially lapped and then scribed to the bottom of the groove so as to leave a portion of the substrate as a projection beneath the bump, thereby facilitating location and joining of the silicon substrate and the detector substrate comprising photodiodes 53. 

From the viewpoint of the mechanical characteristics, glass and silicon resemble each other. They have a similar mechanical hardness, are brittle as they lack plasticity and are thus prone to fracture. Of the standard precision engineering procedures available for shaping, only those that do not use a geometrically defined cutter can be used, such as grinding and lapping. Microengineering techniques are much more efficient, but they do prove difficult for deeper structures. 

The rocking-curve method has been applied to semiconductor materials by several workers. Weissmann(8) estimated 5ft as the depth of damage on germanium lapped with No. 305 abrasive (3200 mesh alundum, having nominal particle size of 5fi). On silicon Andrus and Bond (9) found l/2ft depth for a fine polish, 3ft for No. 305 lap, and 10ft for a diamond saw-cut the widths at half-maximum intensity before any etching were 13<... 

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