Research paperPerformance improvement of commercial ISFET sensors using reactive ion etching
Graphical abstract
Introduction
Complementary Metal Oxide Semiconductor (CMOS) technology has enabled the development of scalable and cheap integrated sensors for biomedical applications. Sensors such as the Ion-Sensitive Field-Effect Transistor (ISFET), used for pH sensing and analysis of various biomolecules such as proteins, enzymes and DNA [1], can be commercialised and integrated in the CMOS process with the extended gate approach [2,3]. This method introduces a passivation layer which, for a typical 0.35 μm process, consists of 1 μm Si3N4 and 1 μm SiO2 which are pH sensitive. Thus, large scale integration of ISFETs in CMOS is possible, enabling the development of novel devices and systems [4].
However, in multi-project wafers (MPW) the final top layer imposed by the CMOS process may be unknown. In the case of CMOS ISFET the top passivation layers are crucial for its functionality and can introduce attenuation of the reference voltage on the floating gate and degrade its sensitivity to pH below the ideal Nernstian of 59 mV/pH [2,3].
The ISFETs were designed in Cadence Virtuoso [5] using the extended gate approach with MOSFET gate oxide thickness 7.6 nm, width 5 μm and length 1 μm. The sensing area is defined by the top metal of 30 μm by 30 μm. The CMOS ISFET chips were fabricated in a commercial 0.35 μm CMOS process. An additional 4 μm thick polyimide layer is typically added during fabrication to release the stress on the materials and to ensure passivation remains intact, since the process has a thick top metal (thicker than the passivation), and it is easier for the passivation to crack. This polyimide layer is not pH sensitive and inhibits the ISFET operation significantly. The final structure and macromodel of the ISFET is shown in Fig. 1, where Eref is the voltage established between the reference electrode and the solution, Vchem is a group of chemically related voltages in the electrolyte solution, Vtc is the voltage referred trapped charge [6], Cgouy and Chelm are the Gouy-Chapman and Helmholtz capacitances of the electrical double layer (EDL) respectively [7] and Cpass is the passivation capacitance [3,8].
We aim to restore pH sensitivity of CMOS ISFET sensors by post-processing using Reactive Ion Etching (RIE) to controllably remove CMOS process-imposed insulation layers and determine the optimum remaining SiO2 layer thickness for sensing.
Section snippets
Experimental
The first exposed layer, polyimide, can be etched with a pure oxygen plasma since volatile carbonyls and water are formed during the plasma process, increasing the etching rate [9]. Mechanical sputtering of the top layer from ions in the plasma will result in a rough surface and cause variations in the distributed capacitance of the passivation layers. This variable capacitance is not desired for good ISFET performance. Additionally, the rougher the top surface the longer the overetch time
Etch characterisation
To evaluate the RIE etching qualitatively we observe the CMOS chips under the microscope at the same light conditions (Fig. 2). The colour difference between the different etch times indicates the change of the top material. For 0 min we see a brown colour that disappears on top of the ISFETs after 10 min of etching. At 10 min etch time, interference fringes on top of the electronics show a non-uniform layer thickness, due to etch inhomogeneity at the edges of mesa structures. The interference
Conclusion
We have shown that by post-processing commercial CMOS ISFET sensors via a non-selective RIE recipe and removing the passivation layers down to SiO2 we boost their pH sensitivity by 125%, passivation capacitance by 5700% and reduction in capacitive attenuation by 96%. The best layer thickness for sensing is found to be 1 μm of SiO2 on top of the gate metal and is achieved with 15 min etching. The trapped charge in the insulation layers increased after RIE but remained constant with the removal of
Acknowledgement
C. Panteli is fully funded by Engineering and Physical Sciences Research Council (EPSRC) doctoral training award.
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Mechanisms for enhancement of sensing performance in CMOS ISFET arrays using reactive ion etching
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