Fabrication

The fabrication process is outlined in Fig. 1.13. The probes are fabricated in parallel using optical lithography. The fabrication process starts with a silicon-on-insulator (SOI) wafer (Fig. 1.13a). First, the height offset between the reference and moving grating fingers to ensure maximum force sensitivity is created using Local Oxidation of Silicon (LOCOS). In this process, an 80-nm-thick layer of stoichiometric silicon nitride film is deposited using low pressure chemical vapor deposition (LPCVD) following an oxidation step to grow a 40-nm-thick oxide layer (Fig. 1.13b). This particular combination of film thicknesses ensures that the tensile stress on the silicon nitride film is compensated by the compressive stress in the oxide layer. Next, the silicon nitride layer is etched down to the underlying oxide layer using plasma etching, and then a thermal oxidation at 1,000°C is performed. During this oxidation process, the dense nitride layer acts as a diffusion barrier for oxygen, and therefore, oxidation of the silicon takes place only in the regions that are not covered by the nitride film, i.e., the reference fingers (Fig. 1.13c). The duration of the oxidation step to achieve the correct height offset is determined using the Deal-Grove model [44]. The advantages of using LOCOS instead of plasma etching to pattern the surface b f i

Tip Etching Afm
Fig. 1.13 Fabrication process for AFM probes with integrated interferometric high-bandwidth force sensors

are that the LOCOS process provides more accurate etch depth control and also preserves the low surface roughness of the Si wafer, which is crucial for the performance of the interferometric grating sensor. Following the LOCOS step, the silicon nitride film is etched in a phosphoric acid at 150°C and the oxide layer is etched using 6:1 Buffered Oxide Etch (BOE) (Fig. 1.13d).

Next, a double side polished Si wafer with a 1-^m-thick oxide layer is fusion-bonded to the patterned device layer of the SOI wafer. The bonding is performed initially at room temperature and is completed by a wet oxidation at 1,050°C for 2 h (Fig. 1.13e). The oxide layer grown during this step is used as a masking material for the subsequent TMAH etching. First, the oxide layer on the substrate of the original SOI wafer is removed using mechanical grinding and the substrate of the starting SOI wafer is etched in a 20% TMAH solution in water at 95°C. Following the etch, the masking oxide layers are stripped using 6:1 BOE.

Note that at this stage of the process, the height offsets for the grating sensor are buried at the interface between the device layer and buried oxide (BOX) layer of the SOI wafer. Next, a 1-^m-thick oxide film is grown at 1,000°C, followed by the deposition of 100-nm-thick silicon nitride film using LPCVD (Fig. 1.13f). To create a masking layer for the tip and thick support regions around the grating sensor, the nitride film and the underlying oxide films are etched using plasma etching and in a 6:1 BOE solution, respectively. Following this, the tips and the thick support regions are created by SF6 based isotropic plasma etching until the oxide/nitride tip masks are released (Fig. 1.13g). Next, the tips are oxide-sharpened at 950°C for 2h (Fig. 1.13h) [45]. The oxide layer grown in the tip sharpening process is used as a masking layer for ion etching the Si device layer to create the cantilever with the grating sensor (Fig. 1.13i). To protect the probe from the subsequent wet etch, the surface is covered with a 1-^m-thick tetraethyl orthosilicate (TEOS) film and then a 300-nm-thick low-stress LPCVD silicon nitride film is deposited as a masking layer for the next KOH etch (Fig. 1.13j). Before KOH etching, the nitride and oxide masking layers on the backside are patterned using plasma etching and 6:1 BOE, respectively. Next, the Si substrate is etched through the backside nitride mask using a 30% KOH solution at 80°C (Fig. 1.13k). Finally, the probes are released by first etching the nitride layer on the front surface using a plasma etch and stripping the remaining oxide masking films in a 6:1 BOE solution (Fig. 1.13l). An SEM micrograph of the finished probe is shown in Fig. 1.14, and a close-up showing details of the interferometric sensor, including the height offset at the base of the reference fingers, is given in Fig. 1.15.

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