**1. Introduction**

Tactile sensation is a highly desired function in the robotics industry [1–9]. Makihata [5] recently reported that the requirements for tactile sensors include large-area sensing capability, which requires a large number of sensors, rapid response time, and low cost.

Tactile sensor arrays are also crucial sensing elements in pulse diagnosis instruments [10–13], which mimic traditional Chinese doctors and determine pulse signals to assess the health of patients. The tactile sensors used for pulse taking must be organized in a large dense array to cover the entire area around the radial arteries, with a sub-millimeter resolution. The integration of sensors with readout circuits is a crucial technology required for large-area, high-resolution sensing.

Extensive research has been conducted to integrate microelectromechanical systems (MEMS) and readout circuits [14–19]. Monolithic integration features low-electronic parasitics, reduced chip pinout, and small size. However, the strict thermal budget and process compatibility results in complex processes and performance tradeoff, which present various problems. Hybrid integration [20–22], which enables MEMS and complementary metaloxide semiconductor (CMOS) devices to be optimized independently, is currently the most

**Citation:** Liu, N.; Zhong, P.; Zheng, C.; Sun, K.; Zhong, Y.; Yang, H. Integrated Piezoresistive Normal Force Sensors Fabricated Using Transfer Processes with Stiction Effect Temporary Handling. *Micromachines* **2022**, *13*, 759. https://doi.org/ 10.3390/mi13050759

Academic Editor: Jose Luis Sanchez-Rojas

Received: 6 April 2022 Accepted: 9 May 2022 Published: 11 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

widely used approach for MEMS and CMOS integration, owing to its short development time, low cost, flexible material selection, and simple fabrication process [23].

Singh et al. [22] proposed a transfer process that can achieve high-density integration by transferring the released MEMS structures onto the readout circuit. Readout circuits can be manufactured using a normal IC foundry and do not undergo etching for release, since the MEMS structures are released before transfer. However, the released MEMS microstructures are movable during the transfer process and can be damaged by shear forces during the bonding and transferring processes. Additionally, the movement of the released MEMS structures decreases the alignment precision of the transferring processes. In a previous study, we proposed a stiction effect temporary handling (SETH) process [24] to temporarily bond the released MEMS structures to the substrates through a stiction effect, which enables temporary handling during the transfer process and reduces alignment errors.

Herein, we present an integrated normal force (force in the z-axis) sensor for pulse diagnosis instruments, wherein the piezoresistive normal force sensor and CMOS readout circuit are integrated using the SETH process. The routing of the piezoresistive Wheatstone bridge can be modified to develop a shear force sensor; consequently, this technique can be used to develop tactile sensors that can sense both normal and shear forces.

### **2. Design and Fabrication**

### *2.1. Design of the Integrated Normal Force Sensor*

In this study, we primarily focused on normal force sensors, since only the force perpendicular to the sensor surface must be measured for pulse taking. The integrated normal force sensor was fabricated by transferring the released force sensor to a CMOS readout circuit chip, as shown in Figure 1. The normal force sensor, which comprises a diaphragm with piezoresistors installed, was fabricated and released through surface micromachining. The diaphragm was suspended by four beams and temporarily attached to the substrate through the stiction effect [25] of surface micromachining to ensure that the normal force sensor did not move during the transfer process, as shown in Figure 1a. The readout circuit chip comprises two CMOS operational amplifiers, which are redistributed to form an instrumentation amplifier. Pads with amorphous silicon/Ti/Au layers on the surface were fabricated on the chip to serve as anchors for the normal force sensor, as shown in Figure 1b. The normal force sensor was then bonded to the readout circuit chip via Au/Si eutectic bonding, as shown in Figure 1c. The diaphragm was transferred to the readout circuit chip after pulling off from the substrate and breaking the suspension beams, as shown in Figure 1d.

The normal force sensor was designed as the sensing element of the pulse diagnosis instrument, which is used in traditional Chinese medicine. A square, flat diaphragm was employed in the normal force sensor, as shown in Figure 2. The size of the low-stress SiN<sup>x</sup> diaphragm was 180 µm × 180 µm × 1.2 µm. The polysilicon layer was heavily doped with boron and patterned with piezoresistors and their interconnections. Cr/Pt/Au electrodes were fabricated on top of the polysilicon layer for eutectic bonding and employed as anchors for the diaphragm after transfer. The size of the diaphragm within the electrodes was approximately 120 µm × 120 µm.

Next, the performance of the normal force sensor was simulated. Two piezoresistors were placed perpendicular to the edge of the diaphragm, while two were placed parallel to the edge. Figure 3 depicts the stresses on the piezoresistors, when 500 µN is loaded on the center of the diaphragm. The force 500 µN is equivalent to 260 mmHg pressure on a 120 µm × 120 µm diaphragm, which is slightly higher than the normal blood pressure. The sensitivity of the normal force sensor was calculated to be 34 µV/µN/V, when the longitudinal and transverse gauge factors of boron-doped polysilicon in [26] were used. The normal force sensor is designed to measure pulse signals, whose frequencies are typically lower than 3 Hz. Because the resonant frequency of the sensor is simulated to be as large as 1.11 MHz, as shown in Figure 4, the sensitivities of pulse signals can be considered equal to the DC sensitivity.

**Figure 1.** Design process flow of the integrated normal force sensor using stiction effect temporary handling (SETH). (**a**) Normal force sensor is fabricated and temporarily attached to the substrate through the stiction effect; (**b**) readout circuit chip is redistributed, and the pads for eutectic bonding are fabricated; (**c**) normal force sensor is bonded to the readout circuit chip; (**d**) diaphragm is transferred to the readout circuit chip after being pulled off from the substrate and broken from the suspension beams.

**Figure 2.** Schematic of the diaphragm of the normal force sensor.

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 4 of 11

equal to the DC sensitivity.

**Figure 3.** COMSOL simulation results for the normal force sensor. (**a**) Txx along the piezoresistor perpendicular to the edge; (**b**) Tyy along the piezoresistor parallel to the edge. **Figure 3.** COMSOL simulation results for the normal force sensor. (**a**) Txx along the piezoresistor perpendicular to the edge; (**b**) Tyy along the piezoresistor parallel to the edge.

large as 1.11 MHz, as shown in Figure 4, the sensitivities of pulse signals can be considered

**Figure 4.** Resonant frequency simulated to be 1.11 MHz by COMSOL. **Figure 4.** Resonant frequency simulated to be 1.11 MHz by COMSOL.

The diaphragm was attached to the substrate after release to improve the alignment precision of flip-chip bonding. The temporary bonding strength produced due to stiction must be lower than the bonding strength of the flip-chip to ensure that the normal force sensor can be successfully transferred to the readout circuit chip. Bumps were fabricated under the electrode to decrease the stiction area, as shown in Figure 1a. The total area of the bump surface was designed to be 2869 μm2, which was much lower than the area of the electrodes (9792 μm2). The normal force sensors can be successfully transferred, even The diaphragm was attached to the substrate after release to improve the alignment precision of flip-chip bonding. The temporary bonding strength produced due to stiction must be lower than the bonding strength of the flip-chip to ensure that the normal force sensor can be successfully transferred to the readout circuit chip. Bumps were fabricated under the electrode to decrease the stiction area, as shown in Figure 1a. The total area of the bump surface was designed to be 2869 µm<sup>2</sup> , which was much lower than the area of the electrodes (9792 µm<sup>2</sup> ). The normal force sensors can be successfully transferred, even if the temporary bonding strength is equal to the eutectic bonding strength.

if the temporary bonding strength is equal to the eutectic bonding strength. To demonstrate integration capability, the normal force sensors were transferred to the CMOS readout circuit chips. The output of the piezoresistive Wheatstone bridge must be amplified using instrumentation amplifiers. Because non-diced wafers of commercial instrumentation amplifiers were unavailable, LMV358 wafers (Yangzhou Genesis Microelectronics Co., Ltd, Yangzhou, China) were employed in our experiments, owing to ease To demonstrate integration capability, the normal force sensors were transferred to the CMOS readout circuit chips. The output of the piezoresistive Wheatstone bridge must be amplified using instrumentation amplifiers. Because non-diced wafers of commercial instrumentation amplifiers were unavailable, LMV358 wafers (Yangzhou Genesis Microelectronics Co., Ltd., Yangzhou, China) were employed in our experiments, owing to ease of accessibility. Two LMV358 amplifiers, considered the CMOS version of the LM358 operational amplifier, were redistributed as 2-op amp instrumentation amplifiers [27], as shown in Figure 5. Amplification was determined using external resistors R1–R4, which presented

resistances of 36, 9.1, 9.1, and 36 kΩ, respectively; the amplification was calculated to be 4.96 using the following equation:

$$V\_o = \left(V\_{in2} - V\_{in1}\left(1 + \frac{R\_4}{R\_3}\right)\right) \tag{1}$$

where *Vin*<sup>1</sup> and *Vin*<sup>2</sup> are the outputs of the Wheatstone bridge. The 3 dB bandwidth exceeded 100 kHz.

**Figure 5.** LMV358 is redistributed to serve as a 2-op amp instrumentation amplifier.

### *2.2. Fabrication*

A normal force sensor was fabricated using surface micromachining processes as follows:


Figure 6 illustrates the redistribution flow of the readout circuit chip, as described below:

(a) The composite layers of SiO2/SiNx/SiO<sup>2</sup> were deposited by PECVD to serve as insulating layers for redistribution. The thickness of each layer was 200 nm. A layer of amorphous silicon of 1 µm thickness was deposited and patterned for subsequent Au–Si eutectic bonding, as shown in Figure 6a.

(b) The contact holes were patterned on the insulating layer and the composite metal layers of Ti/Au were sputtered and patterned to redistribute the operational amplifiers to serve as instrumentation amplifiers, as shown in Figure 6b. The Ti layer was used to decompose native oxide on the surface of amorphous silicon during subsequent Au–Si eutectic bonding [31]. The thicknesses of the Ti and Au layers were 50 and 400 nm, respectively.

**Figure 6.** Redistribution flow of the readout circuit chip. (**a**) After composite layers of SiO2/SiNx/SiO<sup>2</sup> were deposited by PECVD to serve as insulating layers for redistribution, a layer of amorphous silicon was deposited and patterned for subsequent Au–Si eutectic bonding; (**b**) the contact holes were patterned on the insulating layer, and composite metal layers of Ti/Au were sputtered and patterned for redistributing the operational amplifiers to serve as the instrumentation amplifiers.

The released normal force sensor was transferred to the readout circuit chip by flip-chip bonding as follows:

