[0045] Example 1
[0046] Explosives trinitrotoluene (TNT) chemical gas sensor
[0047] In this implementation application, the detection of explosive trinitrotoluene (TNT) gas is taken as an example to illustrate the application of the present invention in chemical gas detection in detail.
[0048] Explosives Trinitrotoluene (TNT) is a commonly used explosive, so it is a very hazardous hazardous material. Effective detection of TNT volatile gases will provide technical support for security inspections and counter-terrorism in transportation hubs and important locations such as airports, stations, ports, customs, etc. It is of great significance to safeguarding public safety.
[0049] The production of a porous silicon cantilever sensor with a size of 300μm×100μm×3μm, see attached figure 2 , The first resonance frequency of the bending mode is about 100kHz, and the specific preparation steps are as follows:
[0050] (a), silicon wafer pretreatment
[0051] The double-sided polished SOI silicon wafer with n-type doped (100) crystal plane is adopted, and the resistivity of the top silicon layer is 0.04~0.15Ω·cm. Based on the given resistivity range, porous silicon with a porosity of about 40% and a pore diameter in the range of 350-500 nm can be obtained. The top layer of silicon is thinned to the thickness of the cantilever beam, which is about 3 microns. Two sides are thermally oxidized to form an oxide layer with a target thickness of about 200 nm, and then a low pressure chemical vapor deposition (LPCVD) method is used on both sides to produce a silicon nitride film with a target thickness of about 100 nm.
[0052] (b) Etching on the back of silicon wafer
[0053] Both sides of the silicon wafer are coated with photoresist with a target thickness of about 1.5 microns. A photoresist is used as a mask, and a pattern that can form a cantilever beam area is photo-etched on the back side. First use reactive ion etching (RIE) to dry etch the silicon nitride layer on the reverse side, and then use buffered hydrofluoric acid to wet etch the exposed silicon oxide, thereby forming an etching window on the back side, using KOH aqueous solution (concentration of 40%, the temperature is 60 degrees Celsius) or other silicon anisotropic etching solutions to etch away the silicon in the window until it stops at the silicon oxide buried layer of the SOI silicon wafer. Remove the remaining photoresist.
[0054] (c) Formation of porous silicon
[0055] A sputtering method is used to first form a titanium-tungsten alloy film with a target thickness of about 50 nm on the back of the silicon wafer, and then sputter again to form a gold film with a target thickness of about 200 nm. Using the same technique as in step (b), the porous silicon area pattern structure window is lithographically etched on the front surface of the silicon wafer, and then a window for exposing silicon for integrated manufacturing of porous silicon is formed on the front surface. Then the metal film is used as the anode, and the platinum electrode in the solution opposite to the front of the silicon wafer and kept at a distance of 5cm is the cathode. Both the anode and the cathode are immersed in the electrolytic cell. The anodizing corrosion solution is HF (40%) and anhydrous C 2 H 5 The OH mixture is subjected to anodic oxidation and corrosion of silicon at room temperature (about 20°C) and a constant direct current. Here, HF in the electrolyte refers to an aqueous solution containing 40% HF (the same below). Experiment selection HF solution and C 2 H 5 The OH is mixed at a volume ratio of 1:1. Attached Figure 4 It is the experimentally measured porosity curve of porous silicon with current density. It can be seen that as the current density increases, the porosity increases; when the current density is higher than 80mA/cm 2 When the porosity reaches 40%. The formed porous silicon undergoes anhydrous C 2 H 5 OH and dilute H 2 O 2 The treatment of the solution is naturally dried in the air. The porosity of porous silicon can be determined by weighing by weight method; the surface morphology, pore shape and pore size of porous silicon can be observed by scanning electron microscope (SEM). Attached Figure 5 The SEM image of the upper surface of the prepared porous silicon is shown, and the current density during the preparation is 150mA/cm 2.
[0056] (d) Micro-machining of cantilever beams
[0057] An etching solution of gold, that is, a saturated solution of iodine in potassium iodide, and an etching solution of titanium and tungsten (such as hydrogen peroxide) are successively used to remove the metal on the back at room temperature. A pattern covering the porous silicon area is formed by spin coating photoresist and photolithography on the front surface of the silicon wafer, and a titanium-tungsten/gold film is formed again by sputtering. The thickness of the titanium-tungsten layer is the same as that of the previous process, but the film thickness this time is thinner than the previous one, at 150 nm, and the light reflected from the gold surface can meet the requirements for resonant signal detection. By wet removing the photoresist in the porous silicon area, the metal film on the cantilever beam can be retained except for the porous silicon area. Then the photoresist is spin-coated and the shape of the cantilever beam is photo-etched. Under the protection of the photoresist, an etching solution is used to remove the gold and titanium-tungsten layers without the protection of the photoresist. Then, RIE dry etching and BHF wet etching are used to successively remove the exposed silicon nitride and silicon oxide. The exposed silicon is deeply etched by the RIE dry method to form a micromechanical silicon cantilever beam structure that can vibrate freely. Finally, the photoresist on the front side of the cantilever beam is removed for use.
