**1. Introduction**

The main advantage of micro light emitting diode (μLED) is that each LED can be controlled and driven independently, leading to excellent power consumption, brightness, resolution, contrast, heat dissipation, and other characteristics [1–3], and μLED has important applications in microelectronics [4], biomedicine [5], and sensor systems [6].

μLED needs to be transferred to the circuit substrate in practical application. Currently, the most popular transfer method is the stamp method, which adjusts the adhesion force by adjusting the peeling parameters of Polydimethylsiloxane (PDMS) to complete the μLED pickup and release. The design of stamp is one of the key technologies of μLED transfer printing [7–9]. A comprehensive understanding of the adhesion between μLED and PDMS is needed to achieve more efficient and high-yield transfer. Therefore, it is very important to measure the adhesion between the μLED and the substrate under different peel speeds and preload.

Recently, many scholars have studied the adhesion between μLED and substrate. Tian Yu et al. found that the peel angle can regulate the adhesion and friction through a theoretical model, which is the mechanism of gecko's strong adhesion and fast separation [10]. Xu Quan, Rogers et al. found that peel speed is an important factor affecting adhesion [11]. Rogers optimized the ground geometry of PDMS by using a sharp substrate, and the strength of adhesion can be switched from strong to weak in a reversible manner by more than three orders of magnitude [9].

**Citation:** Bai, J.; Niu, P.; Cao, S.; Liu, Q. The Adhesive Force Measurement between Single μLED and Substrate Based on Atomic Force Microscope. *Appl. Sci.* **2022**, *12*, 9480. https:// doi.org/10.3390/app12199480

Academic Editor: Andrey Miroshnichenko

Received: 30 July 2022 Accepted: 19 September 2022 Published: 21 September 2022

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<sup>1</sup> School of Mechanical Engineering, Tiangong University, Tianjin 300387, China

Chang Dong Yeo developed an instrument based on capacitive force sensor to measure the dynamic adhesion between rough surfaces [12]. Min sock Kim proposed a new instrument to measure the dynamic adhesion of the interaction surface on the flexible substrate, and proposed an optimal adhesion control strategy based on the analysis of adhesion [13]. In 2017, Lindsay Vasilak used strain gauge load cell to measure the normal adhesive force of OLED [14]. Chang-Dong Yeo used a high-resolution, high-dynamic bandwidth capacitive force transducer and two piezoelectric actuators to measure adhesive pull-off forces between nominally flat rough silicon surfaces under various dynamic conditions [15]. Jaeho Lee used Atomic Force Microscope (AFM) to measure the adhesion between the colloidal probe and silicon wafer. Two spherical colloids made of silicon dioxide and gold that were attached to an AFM cantilever were approached to and retracted from a silicon wafer specimen [16].

However, at present, none has been found in the literature regarding adhesion test between a single μLED and the substrate and most of the experiments demonstrate the adhesion between a large area of PDMS and μLED array, since it is hard to attach a single μLED to the force sensor. In this paper, based on AFM, the adhesion between a single μLED and the substrate was measured using cantilever, and the relationship between peel speeds, preloads, and adhesion was evaluated [17,18]. In Section 2, the theoretical relationship between pull-off forces to peel speeds and preload will be deduced. In Section 3, the theoretical results will be verified experimentally.
