This article presents an analytical modeling of the boron-diffused silicon layers used as a self-stop etching process for the fabrication of the silicon capacitive sensors for biomedical applications by bulk micromachining technology, a key process for an accurate control of the membrane characteristics. Analytical expressions are obtained describing the boron profiles after the diffusion from oxidizing (BBr3, B2O3) and non-oxidizing boron-nitride (BN) source, showing a very good agreement with the experimental data, and which obey two distinct universal curves, independent of diffusion time and temperature. These results are used to calculate the silicon etching rate and time by analytical relations, as a function of the depth and etching temperature in 10% and 24% alkaline-type solutions of KOH (NaOH, LiOH) or ethylene-diamine-based (EDP) solution, allowing an accurate control of the thickness of the silicon membrane and of the processing-induced stress, which finally determines the technical characteristics of the silicon capacitive sensors for biomedical applications.
TopIntroduction
In the present informational era, the informational devices have been increasing their impact not only in individual and global mass media communications (Gaiseanu, 2019a, 2022a), but also in the individual and collective decisional processes (Filip, 2022) by big data analysis (Filip, 2020). The production of the informational devices largely has been grown from simple units to sophisticated integrated circuits and microprocessors, involving for about three decades the intensive development of the microelectromechanical systems (MEMS) (Xu et al., 2019), using these systems to complete the advantages of integration of the sensing informational elements with the micro-processing informational complexes (Song et al., 2020), and of the artificial intelligent systems, which are compatible with human health (Filip, 2021). The individual miniature sensing elements serve in a large range of applications in biomedical, aerospace, automation/automobile fields (Xu et al., 2019; Song et al., 2020), and these are also suitable to be further integrated in more complex systems like “Lab-on-a-chip” (Li, 2008; Lin, 2008), and in silicon-silicon-dioxide-chromium for the detection of the proteins and photo lipids, nitrogen-doped silicon to detect specific proteins, and amorphous silicon-image sensor based on thin-film transistors for X-ray medical imaging. These sensing elements can serve in other suitable combined architecture-complex structures such as silicon probe – polyimide monolithic sensor systems for the detection of neural activity, complementary metal-oxide-silicon (CMOS) and combined bipolar junction transistor with CMOS (BiCMOS), with special biomedical applications for the measurement of the heartbeat rate, of the respiration and of the peripheral and cranial nerve activities. These elements are also used in silicon MEMS-type microphone for the determination of the pulse, silicon nanowires for the detection of deoxyribonucleic acid (DNA) molecules, (Xu et al.,2019), in versatile integration in measurement telematics, together with silicon microprocessors in long-time monitoring medical procedures (Khiem, H. (2011); Xu et al., 2008; Şevket, 1998).
In the healthcare and diagnostic domain, the involvement of the sensing devices and intelligent microsystems became of acute importance, because they offer a compatible and complete intervention due to the used compatible materials, micro-miniaturization and integration into in-situ or in operative Lab-stations micro-processing lines (Gaiseanu, 2019b). The silicon capacitive sensors for biomedical applications are informational microelectromechanical systems (MEMS), measuring the blood pressure, with remarkable advantages related to miniaturization, low-cost production – characteristic to the silicon planar technology, diversity and versatile integration in measurement telematics lines together with silicon microprocessors, in long-time monitoring medical procedures.