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Precision Engineering 41 (2015) 145–152

Contents lists available at ScienceDirect

Precision Engineering

jo ur nal ho me p age: www.elsev ier .com/ locate /prec is ion

echnical note

anometric cutting in a scanning electron microscope

engzhou Fanga,b, Bing Liua,b, Zongwei Xua,∗

State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072, ChinaSchool of Mechanical Engineering, Tianjin University, Tianjin 300072, China

r t i c l e i n f o

rticle history:eceived 25 September 2014eceived in revised form 29 January 2015ccepted 30 January 2015vailable online 12 February 2015

eywords:anometric cutting

a b s t r a c t

A nanometric cutting device under high vacuum conditions in a scanning electron microscope (SEM)was developed. The performance, tool-sample positioning, and processing capacity of the nanometriccutting platform were studied. The proposed device can be used to realize a displacement of 7 �m, witha closed-loop resolution of 0.6 nm in both the cutting direction and the depth direction. Using a diamondcutting tool with an edge radius of 43 nm formed by focused ion beam (FIB) processing, nanometriccutting experiments on monocrystalline silicon were performed on the developed cutting device underSEM online observation. Chips and machining results of different depths of cut were studied during

evicecanning electron microscopenline observationool edge radiusocused ion beam

the cutting process, and cutting depths of less than 10 nm could be obtained with high repeatability.Moreover, the cutting speed was found to exhibit a strong relationship with the brittle–ductile transitiondepth on brittle material. The experimental results of taper cutting and sinusoidal cutting indicated thatthe developed device has the ability to perform multiple degrees of freedom (DOFs) cutting and to studynanoscale material removal behaviour.

. Introduction

Along with the development of cutting technology from con-entional cutting to micro-cutting, even to nanometric cutting, theepth of cut is decreasing and machining accuracy is improving.ue to the size effect, the material removal mechanism of nano-etric cutting may be different from that of conventional cutting.

uan et al. [1] studied the effects of the tool edge radius on theinimum cutting thickness via diamond cutting of Al-alloys, indi-

ating that when the edge radius of the diamond cutting tool was.2–0.6 �m, the minimum thickness of the chips was observed toe 0.05–0.2 �m. Fang et al. [2–5] studied the nanometric cuttingechanism of monocrystalline silicon and found that the chip for-ation in nanometric machining is based on extrusion rather than

hearing. This result indicates that in nanoscale machining, the cut-ing mechanism is significantly different from that in conventionalutting. Malekiana [6] found the minimum cutting thickness was aunction of the tool edge radius and the friction coefficient; depend-ng on the tool geometry and the workpiece property, the average

inimum cutting thickness was 0.23 times the tool edge radius.

u et al. [7] resorted to scratch experiments to study the influ-

nce of plastic machining of monocrystalline silicon with differentrystal orientations, and the phase transformation was analyzed

∗ Corresponding author. Tel.: +86 22 27403753 3.E-mail address: zongweixu@tju.edu.cn (Z. Xu).

ttp://dx.doi.org/10.1016/j.precisioneng.2015.01.009141-6359/© 2015 Elsevier Inc. All rights reserved.

© 2015 Elsevier Inc. All rights reserved.

using Raman spectroscopy. Many research results indicated thatthe conventional cutting theory has been unable to interpret theresults and phenomena effectively during the nanometric cuttingprocess.

In recent years, researchers have focused their efforts on thestudy of the nanometric cutting mechanism, and many remark-able achievements have been obtained. With the developmentof computer technology, research studies on the nanometric cut-ting mechanism were widely implemented by molecular dynamics(MD) simulation [8–11]. To verify the MD simulation results andexperimentally clarify the nanometric cutting mechanism, theatomic force microscopy (AFM) based nanoscratching method[12–15] and the ultra-precision turning based cutting experiments[16–18] were considered.

However, in the study of the nano-cutting mechanism basedon AFM or nanoscratching, although the AFM probe can be quitesharp, the probe or scratch head structure parameters were dif-ferent from the tool parameters, such as the difference in the rakeface and flank face, thereby reducing the validity and authentic-ity of the examined nano-cutting mechanism. Moreover, duringultra-precision turning-based cutting experiments, the depth ofcut was difficult to achieve on the nanoscale and it was difficultto detect when the nanoscale cutting chip comes out. Controlling

the cutting depth to nanometer precision is essential but not suf-ficient. To fully understand the governing physics of nanometriccutting, the cutting process and chip morphology must be observedonline.

146 F. Fang et al. / Precision Enginee

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ig. 1. Schematic of the nanometric cutting device enabling online SEM observation.

The key task in the study of nanometric cutting mechanism iso develop a device for high resolution in situ and online obser-ation of nanometric cutting. The SEM technique has undergoneonsiderable development in recent years because of its high res-lution, with many experiments involving the characterization ofhe properties of nanoscale materials, in situ indentation experi-

ents [19–22], and nano-manipulation experiments [23,24]. FIBechnology has also been widely used in the nano-manufacturingeld, especially in the fabrication of diamond tools with a nano-dge radius and versatile edge profile [25,26]. Therefore, in thisaper, a nanometric cutting device inside an SEM was developednd tested to study the nanometric cutting mechanism. Nano-utting experiments were performed to evaluate the performancef the device.

. Nanometric cutting device

As shown in Fig. 1, the nanometric cutting device is attached tohe sample stage of an SEM, which can be disassembled for testingnd maintenance. The device consists of multi-DOFs microposition-rs, a multi-DOFs nanoscale motion stage, and a nano-manipulator.n addition, the sensors for tool setting, diamond cutting tool, toolandle and other adapting workpieces are included in the device.he main parts of the device are made of stainless steel, whicheets the requirement of being stiff and nonmagnetic.

One of the most important requirements for in situ observation

f the nanometric cutting process is high-resolution SEM imaging.n all experimental processes, both the coarse and fine position-ng are observed via SEM imaging with nanometer resolution. The

able 1ositioning accuracies during different tasks of the nanometric cutting experiments (man

Task Actuator

Coarse positioning of sample Micropositioner

Fine positioning of cutting tool Nanoscalemotion stage

Removal of the chips Nano-manipulator

Positioning the cutting tool to the SEM visual field Beam shift of SEM

Angle adjustment of the sample Stage platform

ring 41 (2015) 145–152

environment of the sample chamber is a vacuum with a vibrationisolated platform that is capable of preventing the effects of for-eign substances and vibration. The function of beam shift in theSEM enables the offset range of ±60 �m via electron deflection ofthe SEM image.

The micropositioner system makes use of the FIB stage, whichbelongs to the FIB dual beam system used in this study. The sys-tem consists of two parts: micropositioner A and micropositioner B.For micropositioner A, there are three DOFs (XYT-axis). For microp-ositioner B, there are another two DOFs (ZR-axis) besides A. Thesample holder is fixed on the micropositioner B, and then, the sam-ple is glued onto the sample holder using liquid carbon conductiveadhesive, avoiding the influence of sticky glue on the stiffness ofthe cutting process. Micropositioner A has a range of movement ofapproximately −20 mm to 20 mm, which is capable of taking thesample rapidly to the machining area, where the gap between thediamond cutting tool and the sample is less than 7 �m. The Z-axis isparallel to the SEM electron beam, which is used to adjust the rela-tive height between the cutting tool and the sample. The R-axis candrive the sample to rotate at a certain angle, which is used for angleadjustment between the cutting direction and sample surface. Therotation T-axis can be used to realize the online observation of dif-ferent angles of the nanometric cutting process via the use of theSEM. Overall, the task of the micropositioner is coarse-positioningthe sample and adjusting the view direction.

A nanoscale motion stage of three DOFs (XYZ-axis), with dimen-sions of 30 mm × 30 mm × 42 mm, is mounted on the isolationplatform for the task of fine-positioning the cutting tool and gen-erating the crossfeed and infeed cutting motions. Each axis of thisstage makes use of a piezoelectric ceramic (PZT) actuator, whichdrives the tool to accomplish a maximum cutting depth of 7 �mwith a closed-loop resolution of 0.6 nm and length of cut of 7 �m.Both the crossfeed stiffness and infeed stiffness of the stage arespecified more than 6 N/�m. The stiffness of the motion stage inthe infeed direction is more important than that in the crossfeeddirection. The total stiffness of the instrument was analyzed accord-ing to finite element analysis, and it is about 2 N/�m in the infeeddirection. The nanoscale motion stage positions the diamond cut-ting tool against the sample exactly under SEM online observation.The diamond tool can be positioned in the X–Y–Z DOFs.

After each cutting experiment, a fraction of the chips remainedon the rake face of the tool, affecting the subsequent observationof the material removal process. For better observation, a MM3Anano-manipulator is used to remove the remaining chips fromthe rake face. Each axis of the nano-manipulator makes use ofan piezoelectric actuator, enabling a step size of 0.5 nm. The posi-tioning accuracies presented in Table 1 proved to be sufficient forthe tasks mentioned. The positioning accuracies are specified asfollows.

3. Performance test

In this study, the nanoscale motion stage driven by a PZTactuator is integrated into the SEM. Due to the creep propertiesof PZT, a closed-loop control system is used to eliminate creep

ufacturer’s specifications).

Travel range Positioning accuracy

X Y Z ˚

±25 mm ±20 �m ±20 �m ±20 �m7 �m ±3 nm ±3 nm ±3 nm±120◦; 12 mm ±0.25 nm 10e−7 rad±60 �m360◦ ±0.1◦

F. Fang et al. / Precision Engineering 41 (2015) 145–152 147

Table 2Comparison of the command and the actual depth of cut. (Average of three repeatedmeasurements).

Command depth ofcut (nm)

Actual depth ofcut (nm)

Maximum errorvalue (nm)

10 10.6 ± 2 2.650 54.9 ± 2 6.9

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Fig. 3. Measurement results of the height difference between adjacent steps(machined surfaces (1) and (2)). The ‘hills’ in between the surfaces (1) and (2) come

100 105.4 ± 2 7.4

isplacement. However, when the depth of cut is on the nano-scale,he most important issues for the device are positioning accuracynd stability. In consideration of the significant influence on theutting results, the stability of the nanoscale motion stage wasnalyzed to ensure that the actual depth of cut is the same as theommand depth of cut.

First, the device developed was placed under working condi-ions for more than 3 h to ensure that the device was stable. Second,

single crystal diamond tool with straight edge was used to cut aingle crystal copper sample, which was processed into a step struc-ure of different depths, as shown in Fig. 2. Single crystal copper wassed as the sample rather than silicon because the large depth ofut may cause the silicon crack, thereby affecting the measurementesults. Finally, the height difference between the two machinedurfaces (1) and (2) was measured using a White Light Interferom-ter. Fig. 3 shows the measurement results of the height difference,.e., the actual depth of cut.

From the results above, it is known that when the commandepth of cut was 10 nm, 50 nm and 100 nm, the measured actualepth of cut was 10.6 nm, 54.9 nm and 105.4 nm, respectively.he comparison of the command depth of cut and actual depths listed in Table 2. The difference between the command depthf cut and the measured depth of cut may result from differ-nt sources, including the hysteresis of the piezoelectric ceramics,lectric noise, measurement uncertainties, etc. The observed differ-nces can be used for compensating the error in the actual depthf cut.

. Nano-cutting experiments under real-time SEMbservation

The nano-cutting experiments were performed in aemperature-controlled metrology laboratory. To better under-tand the cutting process, the fabrication method of the diamond

utting tool and sample must be known.

Fig. 2. Step structure of the single crystal copper sample.

from the surface of the chip adhering to the workpiece.

Fig. 4. Diamond cutting tool shaped by FIB processing.

148 F. Fang et al. / Precision Engineering 41 (2015) 145–152

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Fig. 5. SEM measurement of the tool edge radius.

.1. Diamond cutting tool

A diamond cutting tool with a straight edge was used to performhe nanometric cutting experiments. The device was integratednto a FIB dual beam system. Firstly, the diamond cutting tool wasabricated using FIB processing, and then, the tool was used toerform a series of experiments under SEM online observation totudy the nanometric cutting mechanism. For better observation

f the material removal process, the cutting edge is machined toe straight with the length of 10 �m using FIB processing. In theachining process, the accelerating voltage was 30 kV and the ion

eam current was 100 pA. The diamond cutting tool as fabricated

Fig. 7. Generation process of the cutting

Fig. 6. Rectangular groove structure of the monocrystalline silicon sample.

is shown in Fig. 4. The tool edge radius is approximately 43 nm, asmeasured from the SEM image shown in Fig. 5. The rake angle ofthe tool is 0◦, and the relief angle is 8◦.

4.2. Sample fabrication method

In this study, monocrystalline silicon was used as the samplein all of the experiments described below. To avoid plough inter-

ference between the sample and the tool side surface during thecutting process, the silicon was machined into a rectangular groovestructure to observe the material removal process clearly, as shownin Fig. 6. The width of the rectangular groove structure is slightly

tool shadows on sample surface.

F. Fang et al. / Precision Engineering 41 (2015) 145–152 149

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discontinuous and fractured chips were obtained. This result indi-cates that the cutting at this depth of cut was in brittle mode.

Fig. 13 shows AFM images and SEM micrographs of nano-cutting

Fig. 8. Schematic diagram of the cutting depth error.

ess than the straight tool edge of 10 �m and the length is morehan the range of nanoscale motion stage of 7 �m.

.3. Tool and sample contact

In the nano-cutting experiments, it is important to establish theontact reference between the tool and the sample. To confirmhe tool-sample contact, the micropositioner knob was manuallyotated to move the sample towards the cutting tool until the gapetween the sample and the tool was within less than 3 micronssing the micropositioner. Next, the sample was maintained staticnd the cutting tool was controlled to approach the sample slowlyy using the nanoscale motion stage with visual feedback via SEM

maging. When the tool was close enough to the sample surface, ahadow of the tool would appear on the sample surface. Fig. 7(a–d)hows the generation process of the shadows. Thus, it is very usefulo judge whether the tool is in contact with the sample surface.

Note that if the sample surface is not strictly parallel to theutting direction or cutting edge, then the cutting depth of theachined surface is not consistent. This inconsistency would affect

he analysis accuracy of the nano-cutting results, such as Ramaneasurements, and so on. The differential depth of cut between

he two ends of the machined surface is shown in Fig. 8. In the casehat the straight edge is 10 �m and the cutting distance is 7 �m,he subsequent error of cutting depth �d1 and �d2 is 17.5 nm and2.2 nm, respectively, assuming both � and � are controlled wello within 0.1◦ along the T-axis and R-axis, respectively. When theepth error and the depth of cut are of the same order of magni-ude, a pre-cut procedure was used to ensure the cutting depth ofhe machined surface is consistent.

In addition, it is important to ensure that both the cutting toolnd the convex structure are at the same height along the Z direc-

ion, using the method schematically shown in Fig. 9. First, the-axis was rotated to make the sample and the tool to tilt at a certainngle. Next, the cutting tool was moved to the sample surface alonghe Y direction, producing a tool mark. Subsequently, the distance

Fig. 9. Schematic of the tool positioning in the Z direction.

Fig. 10. SEM photograph of the cutting process. The cutting speed was 23.5 nm/s.

was measured via examination of the cross section. Therefore, theheight difference between the cutting tool and the sample can bedetermined, and then, the nanoscale motion stage can be controlledto make the tool rise or fall a displacement of �Z.

4.4. Nano-cutting experiments

For a cutting experiment, the cutting tool was first broughtin contact with the sample using the technique illustrated above.Once the tool-sample contact was established, the tool was movedalong the reverse cutting direction to the edge of the sample. Next,attempts were made at cutting the sample with a feed of 10 nmevery time. This was performed until the sample was cut, and thelocation at this moment was recorded as the cutting depth of zero.Subsequently, the cutting experiments were started by moving thediamond tool. Figs. 10 and 11 show the cutting process viewed byonline SEM observations. Fig. 12 shows SEM micrographs of thechips formed by using the diamond cutting tool with edge radiusof 43 nm, where the depth of cut was (a) 5 nm, (b) 15 nm, (c) 25 nm,and (d) 40 nm. As seen in Fig. 12(a–c), when the depth of cut wassmaller, the chips formed were continuous. Such continuous chipsobtained at these depths of cut indicated that the cutting was per-formed in ductile mode and that plastic deformation had takenplace. However, as the depth of cut increased to 40 nm (Fig. 12(d)),

with an input sinusoidal trajectory (peak-to-peak value of 50 nm,

Fig. 11. SEM photograph of the entire field of view. The undeformed cutting depthwas (a) 5 nm, (b) 10 nm, and (c) 20 nm.

150 F. Fang et al. / Precision Engineering 41 (2015) 145–152

Fig. 12. SEM micrographs of chips with different depths of cut. The cutting speed was 23.5 nm/s.

Fig. 13. Cutting results with a sinusoidal trajectory half of a full period on single-crystal germanium. The cutting speed was 23.5 nm/s.

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ig. 14. AFM image of a tapered cutting groove on single-crystal silicon. The cuttingpeed was 23.5 nm/s.

ycle of 12 �m) on single-crystal germanium. The cross-sectionalrofile along the X-axis shown in Fig. 13(b and c) correspond tohe machined surface morphology. As seen in Fig. 13(c), the cuttingirection is from left to right. Both plastic cutting and brittle cuttingccurred during this nano-cutting experiment. The critical depthf brittle–ductile transition for single-crystal germanium was

pproximately 20 nm in this particular case (speed of 23.5 nm/s,ry cutting, tool edge radius of 43 nm). Note that the beginningnd end of brittle area appeared at different depths. As shown inig. 13(c), when the cutting tool was controlled from the sinusoidal

Fig. 15. SEM micrographs of machined silico

ring 41 (2015) 145–152 151

trajectory’s peak to valley, compressive stress generated ahead ofthe cutting tool greatly contributed to the ductile machining ofbrittle material. When the cutting tool was controlled from thesinusoidal trajectory’s valley to the next peak, the tensile stress andthe adhesion between the chips and the cutting tool would degradethe ductile machining results.

Fig. 14 shows the AFM image with an increasing depth of cut onmonocrystalline silicon. The depth of cut was from 0 to 30 nm, withthe cutting distance of 7 �m. The experimental results mentionedabove indicate that multi-DOFs cutting can be realized using thenanometric cutting device.

Fig. 15 shows SEM micrographs of the machined surfaceobtained in cutting silicon, where the depth of cut was 24 nm andthe cutting speeds were (a) 1.4 mm/s, (b) 117.6 nm/s, (c) 23.5 nm/s.The figure shows that at cutting speeds (a) and (b) the surface wasfree from cracks. However, when the cutting speed was reduced to23.5 nm/s (c), some brittle cracks appeared on the silicon surface.This result indicates that the higher the cutting speed, the better thesurface quality. In addition, compared to the results in Fig. 12(c),the cutting speed significantly affected the brittle–ductile transi-tion depth. Further study of the influence of the cutting speed willbe addressed in another paper.

5. Conclusions

A new nanometric cutting device that employs a nanoscalemotion stage to provide accurate nano-cutting motions was devel-oped. The device used inside an SEM can realize the online

observation of the nano-cutting process, which can be usedto effectively study the nanoscale material removal mecha-nism experimentally. The fundamental performance of the devicewas investigated. A method of confirming tool-sample contact

n surfaces at different cutting speeds.

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as introduced. The experimental results of nanometric cuttingemonstrated that the developed device has the ability to per-orm multi-DOFs cutting and enables nanoscale material removalehaviour to be achieved.

cknowledgments

The authors appreciate the support of the Nationalasic Research Program of China (973 Program, Grant No.011CB706700), the National Natural Science Foundation ofhina (Grant Nos. 91423101 & 51275559), LPMT, CAEP (Granto. KF13008), and the ‘111’ project by the State Administrationf Foreign Experts Affairs and the Ministry of Education of ChinaGrant No. B07014). The authors thank Mr. Wei Wu for support inhe experiments.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precisioneng.015.01.009.

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