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Investigation on the Edge Chipping in Ultrasonic Assisted Sawing of Monocrystalline Silicon

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Investigation on the Edge Chipping in Ultrasonic Assisted Sawing of Monocrystalline Silicon micromachines Article Investigation on the Edge Chipping in Ultrasonic Assisted Sawing of Monocrystalline Silicon Jianyun Shen, Xu Zhu * , Jianbin Chen, Ping Tao and Xian Wu College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China; [email protected] (J.S.); [email protected] (J.C.); [email protected] (P.T.); [email protected] (X.W.) * Correspondence: [email protected]; Tel.: +86-1870-297-7299 Received: 14 August 2019; Accepted: 13 September 2019; Published: 16 September 2019 Abstract: Monocrystalline silicon is an important semiconductor material and occupies a large part of the market demand. However, as a hard-brittle material, monocrystalline silicon is extremely prone to happen edge chipping during sawing processing. The edge chipping seriously affects the quality and performance of silicon wafers. In this paper, both conventional and ultrasonicassisted sawing tests were carried out on monocrystalline silicon to study the formation mechanism of edge chipping. The shape and size of edge chipping after sawing process were observed and measured. The experimental results demonstrated that different sawing processes present different material removal modes and edge quality. The mode of crack propagation was continuous cracks in conventional sawing process, while the expansion mode in ultrasonic assisted sawing was blasting microcracks. This results in the cutting force of ultrasonic assisted sawing becomes much smaller than that of conventional sawing process, which can reduce the size of edge chipping and improve the quality of machined surface. Keywords: monocrystalline silicon; ultrasonic assisted sawing; edge chipping; edge chipping mechanism 1. Introduction As an important semiconductor material, monocrystalline silicon is widely employed in electronics, communications and energy field [1–4]. With the development of semiconductor material technology, there are increasing demands on the performance and quality of silicon wafers, and the proportion of large-diameter silicon wafers that is suitable for micro fabrication in the market is increasing as well. At present, the production process of monocrystalline silicon components includes pulling, wire cutting, grinding, polishing, and dicing [5–8]. The dicing is the last step before packaging [9,10]. As a typical hard-brittle material, monocrystalline silicon is easy to produce brittle cracks and damages during the cutting process [11–13]. The occurrence of edge chipping will greatly reduce the performance and quality of semiconductor products. The sawing process of silicon wafer with diamond saw blade is a common cutting method. Efra et al. [14] pointed out that the phenomenon of edge chipping in the cutting process is one of the three main factors that reduce production efficiency. Lin et al. [15] discovered that the surface features of saw blade were an important factor Affecting the phenomenon of edge chipping. Luo et al. [16] explored the edge chipping mechanism through studying the relationship between sawing direction, crystal direction and chip size during cutting of monocrystalline silicon with diamond saw blade. Moreover, Cvetkovic et al. [17] found that the ultra-precision cutting with the saw blade has many unique advantages for suppressing edge chipping and reducing sidewall roughness compared to wire sawing, but it is easy to cause greater sidewall curvature. As an advanced machining technology, the ultrasonic vibration assisted sawing is widely used to machine hard-brittle materials [18–21]. It not only improves the surface quality after machining, Micromachines 2019, 10, 616; doi:10.3390/mi10090616 www.mdpi.com/journal/micromachines Micromachines 2019, 10, x FOR PEER REVIEW 2 of 13 MicromachinesAs an advanced2019, 10, 616 machining technology, the ultrasonic vibration assisted sawing is widely 2used of 13 to machine hard-brittle materials [18–21]. It not only improves the surface quality after machining, but also increases the machining efficiency [22–26]. In addition, the sawing force during processing butcan alsobe reduced, increases which the machining is helpful to effi reduceciency edge [22–26 chipping]. In addition, [27–31]. the Wang sawing et al. force [32] during designed processing a novel canexperimental be reduced, method which and is helpful studied to the reduce characteristics edge chipping of edge [27– chipping31]. Wang in et rotary al. [32 ultrasonic] designed drilling. a novel experimentalShen et al. [33] method developed and a studied radial ultrasonic the characteristics vibrating of shank edge that chipping could inbe rotary mounted ultrasonic directly drilling. on the ShenCNC etcenter, al. [33 and] developed explored athe radial groove ultrasonic morphology vibrating after shank sawing that and could the characteristics be mounted directly of the sawing on the CNCforce. center,Nowadays, and explored ultrasonic the vibration groove morphologyassisted sawi afterng has sawing become and an the important characteristics research of direction the sawing in force.the field Nowadays, of micro fabrication ultrasonic vibrationof monocrystalline assisted sawing silicon has wafers. become an important research direction in the fieldIn this of micro paper, fabrication ultrasonic of assisted monocrystalline sawing technique silicon wafers. was taken advantage to reduce the edge chippingIn this of paper,monocrystalline ultrasonic silicon. assisted The sawing formatio techniquen mechanism was taken of edge advantage chipping to reduce during the sawing edge chippingprocess was of monocrystalline analyzed in detail. silicon. Effects The formation of ultras mechanismonic assisted of edge vibration chipping on during the edge sawing chipping process wasphenomenon analyzed during in detail. sawing Effects were of ultrasonic studied by assisted monitoring vibration the onsawing the edge force, chipping measuring phenomenon the edge duringchipping sawing size and were observing studied the by monitoringedge morphology. the sawing force, measuring the edge chipping size and observing the edge morphology. 2. Kinematic Characteristics of an Abrasive Particle in Ultrasonic Assisted Sawing 2. Kinematic Characteristics of an Abrasive Particle in Ultrasonic Assisted Sawing Ultrasonic assisted sawing can be seen as a combination of conventional sawing and ultrasonic vibrationUltrasonic processing. assisted As sawingshown canin Figure be seen 1, in as the a combination ultrasonic-assisted of conventional sawing, the sawing motion and of ultrasonic abrasive vibrationgrains on processing.the saw blade As shownincludes in a Figure rotational1, in themotion, ultrasonic-assisted an ultrasonic sawing,vibration the perpendicular motion of abrasive to the grainsworkpiece, on the and saw a motion blade includes in the feed a rotational direction. motion, According an ultrasonic to our previous vibration study, perpendicular the trajectory to the of workpiece,single abrasive and grain a motion in ultrasonic in the feed assisted direction. sawing According can be to expressed our previous as [34]: study, the trajectory of single abrasive grain in ultrasonic assisted sawing can be expressed as [34]: 2 2 sin + + sin( 2) sin () ! 0 2πωt 2πωt 1 l (t) B r sin 60 + v!t + A sin(2π f t) sin 60 C ()( ) = = ux == B 60 60 C (1) lu t () B 2 2πωt 2πω2t C (1) luy(t) 1−cos@ r 1 cos −Asinsin((2 2)π f t) cos cos A − 6060 − 60 60 where A is the ultrasonic amplitude, ff isis the the ultrasonic ultrasonic vibration vibration frequency, frequency, vw isis thethe feedfeed speed of workpiece, ω! and rr are the angular velocity and radius of the saw blade, respectively, and t is the machining time. Figure 1. Illustration of ultrasonic-assisted sawing. ( (aa)) basic basic configuration; configuration; ( (b)) the motion of single abrasive grain. The trajectory schematic diagram when single abrasive grain rotating drawn using MATLAB software (R2014b,(R2014b, MathWorks,MathWorks, Inc., Inc., Natick, Natick, MA, MA, USA) USA) according according to to Formula Formula (1) (1) is shownis shown in Figurein Figure2a. As2a. illustratedAs illustrated in Figurein Figure2a, 2a, the the cutting cutting distance distance of of grains grains in in the the ultrasonic ultrasonic assisted assisted sawing sawing is is longerlonger compared with conventional sawing in the same time. It is meant that abrasive grains in ultrasonic assisted sawing have greater speed compared to conventionalconventional sawing.sawing. Furthermore, the interaction relationship between abrasive grains and the workpi workpieceece is shown in Figure 22b.b. ItIt cancan bebe foundfound thatthat the abrasive grains intermittently contact with the workpiece surfacesurface duringduring cutting.cutting. That means the abrasive grains repeatedly cut-in and hammer the workpiece surface in the cutting zone of cutting Micromachines 2019, 10, x FOR PEER REVIEW 3 of 13 Micromachines 2019, 10, x FOR PEER REVIEW 3 of 13 Micromachinesabrasive grains2019, 10repeatedly, 616 cut-in and hammer the workpiece surface in the cutting zone of cutting3 of 13 arcabrasive with ultrasonicgrains repeatedly frequency. cut-in Effective and hammer contact thetime workpiece te can be surfacecalculated in theaccording cutting tozone the ofstudy cutting by Wangarc with et al.ultrasonic [35]. frequency. Effective contact time te can be calculated according to the study by arc with ultrasonic frequency. Effective contact time t can be calculated according to the study by Wang et al. [35]. e Wang et al. [35]. 1 2 r (2) = 2d 11 2p te ==
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