Figure 1 shows the structure of the integrated sensor. The micro-force sensor consists of the flexible sensing structure, the overload protection glass and the quartz optical fiber probe. Four sensing beams are used to support the central mesa in the flexible sensing structure. One glass optical fiber (GOF) probe is glued on the flexible sensing structure as the tactile probe. The glass bonded to the bottom of the sensor chip is the overload protection unit, and the central mesa-to-glass gap is 5 μm. When the sensor is overloaded, the central mesa will get in touch with the glass that could protect the cantilever beams from breaking. Force can be applied to the probe in the X, Y or Z orientations, which then leads to beam deformation as shown in Figure 2 which is simulated by ANSYS. Dimensions of the micro-force sensor are shown in Figure 1(b), the thickness of beams and pressure sensor membrane are all 35 μm. When a force is applied on the probe the membrane deforms, creating a maximum strain near both ends of the cantilever beams. Hence twelve p-type piezoresistors in the form of three Wheatstone bridges are placed on the sensing beams at the locations where the maximum strain is generated. The changes of the piezoresistors caused by strain are linearly proportional to the applied force.

For piezoresistive silicon micro-sensor, the maximum strain of resistors should be smaller than 500 micro strains [18]. In the range of strain, the sensor could achieve high linearization and sensitivity. The maximum strain, in Figure 2(a), is 523 micro-strains, so when the applied force in the X or Y orientation, is smaller than 3 mN, the micro-force sensor in the X or Y orientation could achieve high linearization and sensitivity, while in Figure 2(b), the maximum strain is only 51.1 micro-strains that could guarantee high linearization of the sensor in the Z orientation.

The absolute pressure sensor is designed as a square membrane structure. Pyrex glass is bonded to the bottom of the silicon wafer to form the vacuum chamber. When a vertical pressure is applied on the sensor, the membrane deflects vertically down, as shown in Figure 3(a). Four p-type piezoresistors are formed on the square membrane. Piezoresistors in the form of Wheatstone bridges are placed near frame of the membrane where maximum strain is generated, as shown in Figure 3(b). Due to the strain in the membrane the resistivity of the p-diffused piezoresistor begins to change. The mechanical signal can be converted into electrical signals by using the imbalance of the excited Wheatstone bridge circuits. To achieve good linearity, the biggest deflection of the membrane must be smaller than a fifth of the membrane thickness and the maximum strain of the resistors should be smaller than 500 micro strains. Due to the sensor measurement range is 0–200 KPa, the maximum deflection and strain of the membrane, 200 KPa pressure applied on the sensor, are calculated by the software ANSYS and shown in Figure 3. It is proved that the maximum deflection is only 0.364 μm, shown in Figure 3(a), which is much smaller than fifth of membrane thickness (35 μm, shown in Figure 1(b)). The maximum strain is only 229 micro strains, shown in Figure 3(b), which is much smaller than 500 micro strains, so we conclude that the absolute pressure sensor can achieve high linearization.

The thermistors of the temperature sensor and the piezoresistors of force sensor and pressure sensor, formed by ion implanting, are all sensitive to the temperature and pressure simultaneously. To guarantee the accuracy of these sensors, we have to make huge efforts to minimize the cross interference.

There two kinds of silicon resistors, p-type silicon resistor (p-Si formed on the n-type silicon wafer by boron ion doping) and n-type silicon resistor (n-Si formed on the P-type silicon wafer by phosphorus ion doping). As shown in Table 1 [19], p-Si in [11̄0], [110] crystal orientation and n-Si in [100], [010] crystal orientation are both suitable for accelerometer and pressure sensors with their their high piezoresistance coefficient, while only p-Si in [100], [010] crystal orientation is suitable for temperature sensoras due to its low piezoresistive coefficient, so the (100) n-type SOI wafer is selected to fabricate the multi-sensor chip to form p-type silicon resistors.

In the range of BC (−55 °C–175 °C), the resistivity of p-type silicon resistors increases linearly proportional to the temperature [20]. Figure 4 shows the piezoresistance coefficient (denoted as P) of the resistors according to different doping concentration (denoted as N) and temperature (denoted as T) [21]. Depending on the temperature increasing, the piezoresistance coefficient decreases, in the range of 0–3 × 10¹⁹ cm⁻³ boron ion doping concentration. At some stable temperature in the range of −75 °C –150 °C, when the boron ion doping concentration is more than 3 × 10¹⁷ cm⁻³, the piezoresistance coefficient decreases gradually.

Sensitivity of the force sensor and pressure sensor can be adjusted by varying the boron ion doping concentration. Lower boron ion doping concentration gives a higher piezoresistive coefficient and higher sensitivity, but also higher temperature interference, while in the range of 3 × 10¹⁸ cm⁻³ and 5 × 10¹⁸ cm⁻³ boron ion doping concentration, the temperature interference for resistor and piezoresistive coefficient can mutual offset, so to get high sensitivity and low temperature interference, 5 × 10¹⁸ cm⁻³ is selected as suitable boron ion doping concentration to form force sensor and pressure sensor resistors. Some methods are employed to minimize interference of the temperature sensor resulting from the applied pressure: high boron ion doping concentration 2 × 10²⁰ cm⁻³, with low piezoresitive coefficient (shown in Figure 5), is selected to form the thermistors; the thermistors are located at the surface of silicon wafer above the silicon-glass bonding area where the strain resulting from the applied pressure is small; the thermistors are designed as wave type structures arranged along the [100] and [010] crystal directions, shown in Figure 1, which achieves the minimum piezoresistive coefficient of the (100) silicon.

Silicon micromachining technology is used to fabricate the sensor chip. The process starts with a double-side-polished four-inch N-type (100) oriented single-crystal Si wafer (Figure 6(a)). The fabrication steps are as follows (Figure 5): (1) Grow an oxide (SiO₂) layer on the surface of the Si wafer by thermal oxidation in a heating oven (Figure 5(b)); (2) Pattern the piezoresistors and piezoresistors contact area to the oxide surface by photolithography (Suss MA6), then form them by boron ion doping in an ion implantation system (Figure 5(c,d)); (3) two layers of SiO₂/silicon nitride(Si₃N₄) bi-material film, which can reduce the effect of the thin film stress in the process of film fabrication, are deposited by low-pressure chemical vapor deposition (LPCVD ASM LB35) to be used as two layer masks for wet anisotropic etching on the backside of wafer and to protect the resistors on the front-side of wafer from being corroded (Figure 5(e)); (4) Pattern the back structure of the integrated sensor to the bi-material film surface by photolithography, and remove the Si₃N₄ layer by phosphoric acid (H₃PO₄) solution, remove the SiO₂ layer by buffered hydrofluoric acid (BHF) solution (Figure 5(f)); (5) The backside of the integrated sensor chip is etched in KOH solution to about 365 μm (Figure 5(g)); (6) Pattern the bottom of the central mesa to the bi-material film surface by photolithography, remove the last SiO₂/Si₃N₄ layer (Figure 5(h)); (7) The backside of the silicon wafer is immersed in KOH solution again to form the narrow gap (5 μm) between the bottom of central mesa and Pyrex glass, complete the final formation of the backside of the integrated sensor (Figure 5(i)); (8) Pattern the contact hole to the bi-material film surface of the silicon wafer front side by photolithography, and remove the SiO₂/Si₃N₄ by reactive ion etching (RIE) (Figure 5(j)); (9) Ti/Al bi-layer film is deposited by physical vapor deposition (PVD, Varian 3180 ion sputtering coator) (Figure 5(j)), and then the metal wire is etched by RIE, Figure 6(c) shows the distribution of the metal wire; (10) The cantilever beams of the force sensor are formed by Inductively Coupled Plasma (ICP, STS ICP ASE) etching; (11) Cr/Au bi-layer is deposited on the Pyrex glass, as the metal electrode, to avoid the electrostatic adhesion between the central mesa and Pyrex glass (Figure 5(k)); (12) The silicon and glass is bonded by the anodic bonding technology (Suss SB6 VAC Substrate Bonder, Figure 5(k)); (13) Residual stress of the fabrication process is released by annealing. The finished sensor chip is shown in Figure 6.

As the tactile element of the micro-force sensor, the probe translates the applied force to the sensor chip. It's very critical to design and fabricate the probe to obtain high sensor sensitivity and accurate measurements. The probe is designed as a ladder structure to get a sharp tip and broader base. With the sharp tip, the micro-force sensor can measure the applied force more accurately. For example, when the probe tip scans over a specimen, which has a small hole on its surface, a sharp tip will sense this hole, while a blunt tip will not [22]. The broader base of the probe means a larger contact area, which is beneficial to keep the probe more securely glued on sensor chip and make the measurement range be larger in the X and Y orientation. Also with the ladder structure, the dead weight of the probe should be lighter and output voltage of the force sensor with no force applied should be smaller.

Over decades, GOF has been used in many fields for measuring physical properties or operating objects. GOF is selected as the probe because of its light weight, immunity to electromagnetic interference (EMI), small size, high sensitivity, and capability of operating at high temperatures [17]. As GOF are made with 99% of silica glass, the Young modulus of the probe is close to that of silicon chip, specifically, the former is 73 GPa (for silica), while the latter is 130 Gpa [for silicon in (100) crystal face]. The coefficient of thermal expansion of silica is only 0.55 × 10⁻⁶/°C, which can minimize the temperature effects when the sensor works under different temperatures.

The GOF has a cladding around 125 μm in diameter that can be etched by buffer oxide etch (BOE) solution, which contains a mixture of HF and NH4F. The GOF tip is dipped into the BOE solution filled in a plastic container in the water bath at 40 °C. The finished probe tip is shown in Figure 7(a). The sharp tip is only 47.81 μm in diameter.

Once the desired diameter of the fiber is obtained after etching, the next step is to properly glue the etched fiber on the micro-force sensor chip and fixed the integrated sensor chip on the PCB to finish the package of the multifunction micro-sensor.

The transparent organic glass shell which is a piece of acrylic, with a hole on it, is used for probe position and fixed. The packaging steps are as follows: first, place the sensor chip in the blind hole of the PCB and glue them together by epoxy resin adhesive, solidification occurs at 60 °C for two hours; then, drop a little epoxy resin adhesive on the central mesa of the micro-force sensor chip and place the transparent organic glass over the PCB, the probe is fixed on the central mesa through the hole of the transparent organic glass; at last, after solidification, the shell is taken off from the PCB and down-lead the micro-sensor chip by gold wire bonder, the finished sensor is shown in Figure 7(b).