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Scence Arts & Méters (SAM) s an open access repostor that collects the work of Arts et Méters ParsTech researchers and makes t freel avalable over the web where possble. Ths s an author-deposted verson
Scence Arts & Méters (SAM) s an open access repostor that collects the work of Arts et Méters ParsTech researchers and makes t freel avalable over the web where possble. Ths s an author-deposted verson publshed n: Handle ID:. To cte ths verson : Emmanuel BROUSSEAU, Benoît ARAL, Stéphane THIERY, Erc YIRI, Olver GIBARU, J.Rhett MAYOR - Towards CC Automaton n AFM Probe-Based ano Machnng - In: 8th Internatonal Conference on McroManufacturng (ICOMM 0), Canada, Proceedngs of the 8th Internatonal Conference on McroManufacturng (ICOMM) - 0 An correspondence concernng ths servce should be sent to the repostor Admnstrator : Towards CC Automaton n AFM Probe-Based ano Machnng E.B. Brousseau, B. Arnal, S. Ther, E. r 4, O. Gbaru 5 and J.R. Maor 6 Cardff School of Engneerng, Cardff Unverst, UK; ICOMM 0 95 Arts et Meters ParsTech, France; LSIS, Arts et Meters ParsTech & O-A IRIA-Llle ord Europe, France; 4 LSIS, Arts et Meters ParsTech, France; 5 LSIS, Arts et Meters ParsTech & O-A IRIA-Llle ord Europe, France; 6 Woodruff School of Mechancal Engneerng, Georga Insttute of Technolog, USA; Ke Words: AFM probe-based machnng, CAD/CAM, automaton, nanomanufacturng ABSTRACT Ths paper presents a feasblt stud, whch ams to demonstrate the applcablt of the CC automaton phlosoph for the process of AFM probe-based nano machnng conducted on commercal AFM nstruments. In partcular, t s proposed to machne n ths wa nanostructures generated wth an CAD software va the representaton of tp path trajectores wth G-code nstructons. Such a representaton can then be nterpreted wth a post processor at the nterface of an AFM nstrument. To demonstrate the valdt of the proposed approach, t was mplemented on a comple pattern. The results obtaned open further research perspectves wth respect to mnmng the sources of machnng errors observed. ITRODUCTIO Although the Atomc Force Mcroscope (AFM) was orgnall developed for the purpose of magng and characterng specmens at the nano scale [], numerous researchers have used such an nstrument n the last 0 ears as a platform for varous nano fabrcaton tasks []. As a result, AFM probe-based technques have been proposed to enable processes such as nano manpulaton [], deposton [4] and removal of materal [5]. Ths latter technque for structurng surfaces at the nano scale reles on the mechancal modfcaton of materal caused b the drect contact between the tp of an AFM probe and the sample surface as shown n Fg.. Fg. : AFM probe-based machnng prncple More specfcall, when performng ths process, the tp of an AFM probe s brought nto contact wth a sample/workpece untl a predefned load s reached. Then the workpece s moved relatve to the tp va lnear peoelectrc actuators, whch generate dsplacements of the stage along the X and Y aes wth sub-mcrometer resoluton. Accordng to Hooke s law, the normal force appled b the tp on a workpece depends on the deflecton of the probe s cantlever n the normal drecton at the tp poston. Ths deflecton s normall measured b projectng a laser beam on the cantlever and b montorng the dsplacement of the beam reflecton wth a photodetector. The photodetector output sgnal can be used to mplement a feedback loop to mantan a constant cantlever deflecton as the tp moves across the sample surface. Ths s acheved b adjustng constantl the vertcal dsplacement of the probe wth a lnear peoelectrc actuator on whch the probe s attached. The attractve characterstc of AFM probe-based mechancal machnng s that the process s relatvel smple and low-cost to mplement [6]. In addton, t has shown hgh fleblt n producng comple three dmensonal (D) features and has been appled for cuttng a wde range of engneerng materals such as metals, semconductors and polmers [7]. Gven that AFM nstruments were, and stll are, prmarl desgned for magng tasks, b default, the tpcal path followed b the tp of an AFM probe mplements a raster scan strateg. Although some AFM manufacturers provde software modules to perform lthograph operatons, such solutons can be lmted wth respect to ) the range of tp motons that can be developed, ) the fleblt n realng purposel-defned tp dsplacement strateges and ) ther portablt for easl transferrng trajectores data between dfferent AFM nstruments. As a result, n order to conduct partcular AFM-based nano fabrcaton operatons man researchers have had to mplement customed procedures and computer routnes enablng the realaton of a large varet of tp trajectores. In ths contet, t would be advantageous for future tp-based nano fabrcaton studes to develop more automated, portable and fleble solutons that could enable ) the path of AFM tps to be defned va wdespread desgn software tools and ) the mplementaton of such tp trajectores to be con- correspondng author ducted on a broad range of AFM nstruments. Unfortunatel, efforts n ths drecton have onl been conducted b a few researchers n recent ears. In partcular, Horkas and co-workers mplemented an AFM software wth bult-n functonaltes for lthograph applcatons [8]. In ths work, a specfcall-desgned CAD nterface was developed to enable the drawng of tp trajectores b the user, whch could subsequentl be sent to the controller of an AFM nstrument. In [9], the authors developed an AFM nanolthograph software wth a purposel-bult graphcal user nterface for desgnng patterns. An attempt was made at mprovng the portablt of stored tp trajector data b representng them as functons n a Wndows meta fle. Other authors proposed a soluton that lnked a commonl used CAD software wth an AFM [0]. To acheve ths, a CAM software was modfed to read CAD drawngs of nano patterns and translate them nto propretar tp trajector data. A common drawback of these proposed approaches s that the do not offer a soluton where the generaton of the machnng paths can be acheved n a full automated manner whle enablng the desgn and path plannng steps to be carred out wthout purposel-bult software tools. In contrast, Johannes et al. mplemented a nano scale desgn envronment for AFM anodaton b ncorporatng conventonal CAD/CAM software [, ]. The nterestng aspect of ths work s that the G-code fle format was used to communcate the tp paths to an AFM controller. In the case of materal removal operatons performed wth the tp of an AFM probe, the most promsng approach to enable ncreased fleblt and automaton of the desgn and machnng tasks s that reported above b Johannes and co-workers. In partcular, gven the maturt of estng CAD/CAM solutons whch enable the seamless ntegraton of D modelng and tool path plannng steps n conventonal cuttng processes, t s natural to reuse such an approach for the purpose of automatng AFM probe-based nano machnng tasks. Besdes, ths approach could contrbute to the development of a more fleble and portable soluton for nanofabrcaton tasks whch would not be restrcted to partcular customed software or AFM nstruments. In ths contet, the purpose of the research presented n ths paper was to follow the method put forward b Johannes and co-workers n the contet of nano machnng operatons rather than anodaton lthograph. In partcular, the objectve of ths research s to demonstrate the feasblt of establshng a lnk between CAD and AFM probe-based nano machnng va the development of a G-code post processor for the AFM equpment controller that wll nterpret the G-code representaton of tp path trajectores generated usng CAM software. CAD/CAM APPROACH FOR AFM PROBE-BASED AO MACHIIG A. METHODOLOGY Fg. llustrates the CAD/CAM approach followed n ths CAD software Fg. : CAD/CAM approach adopted stud. It s proposed that the D models of the nano structures to be machned can be desgned usng an conventonal CAD software. et, a neutral fle format, such as the Intal Graphcs Echange Specfcaton (IGES) data format, can be utled to transfer desgn data to a CAM software. Tp path trajectores can then be automatcall defned wth the path generaton functonaltes provded b such software. In partcular, parameters such as the dstance between two grooves (.e. the step-over), the cut drecton, the tolerance and the tool geometr are provded b the user when defnng the tp paths. The data created n ths wa can then be represented b a set of G-code nstructons and be communcated to the controller of an AFM nstrument. G-code s a wdel used computer numercal control (CC) programmng language and thus, t s mplemented b commonl found CAM software. When deplong ths approach wth a partcular AFM nstrument, t s necessar for the user to develop a post-processor that can translate the G-code format nto nstructons that can be understood b the AFM controller. Ths s requred as conventonal AFM sstems have not been developed wth the prmar purpose of conductng nano machnng operatons and understandabl, the do not have bult-n capabltes to read G-code nput. B. IMPLEMETATIO In ths work, the partcular CAD and CAM software used were SoldWorks and PowerMll respectvel. The AFM nstrument utled was the XE-00 model from Park Sstems. A post-processor was developed to translate G-code data nto nstructons for the AFM controller usng C++ lbrares provded b Park Sstems. The flow of data processng from the CAD software to the AFM controller s llustrated n Fg.. The probes emploed to perform the machnng eperments were DISP probes from Bruker (see Fg. 4). Ths tpe of AFM probe s normall emploed for nano-ndentaton eperments. It s made of a cantlever n stanless steel on whch a three sded damond tp s glued. The partcular AFM probe emploed had a nomnal normal sprng constant of.m - and the nomnal tp radus specfed was 40 nm. CAD software SoldWorks AFM controller XE-00 IGES fle CAM software IGES fle Propretar nstructons G-code fle CAM software G-code fle PowerMll AFM controller Post processor C++ programme Fg. : Implementaton of the CAD/CAM approach adopted 5µm Startng pont A Bresenham s segments Desred lne Fg. 4: AFM probe utled The workpece processed was a dual phase brass allo CuZn9Pb wth dmensons mm mm mm. Usng wre electro dscharge machnng, ths specmen was cut from an ntal 4 nch brass crcular wafer prepared wth a successon of lappng and polshng steps. The surface roughness acheved n ths wa was Ra 0 nm as measured wth a whte lght nterferometer (McroXAM-00-HR). The hardness of each phase present n the brass allo was also measured usng a mcro hardness tester (Mtutoo Mcro-Vckers Hardness Tester HM-). In ths case, an average was calculated from fve measurements conducted n each phase separatel under a load of 0 g. For the α and β phases, the hardness was ~5 HV and ~ 0 HV respectvel. It was decded to carr out the cuttng operatons onl on grans correspondng to the α phase n order to avod changes n the processng condtons that can be ntroduced from processng both phases smultaneousl. In partcular, t s known that for mcro- and nano-scale cuttng, the crstallne structure of processed materals has a sgnfcant nfluence on dfferent machnng characterstcs [, 4]. The ntal development stage of the proposed approach revealed that ts mplementaton wth the partcular AFM nstrumentaton used had two mportant constrants. The frst one s the fact that the AFM controller could onl generate the stage lateral dsplacements along four aes, namel n the drectons perpendcular, parallel and at ± 45 degrees angle wth respect to the orentaton of the long as of the cantlever. Thus, to eecute G-code nstructons between ponts as accuratel as possble, the lnes or curves representng the planned tp trajectores have to be dscreted nto smaller segments orented along one of the four constraned drectons mentoned above. Ths dscretaton step was acheved wth the Bresenham s lne algorthm, whch s commonl used n computer graphc applcatons. Fg. 5 llustrates the mplementaton of ths algorthm for appromatng a lne whch does not follow one of the aes of constraned dsplacements. Fg. 6 shows the results of mplementng ths approach when machnng grooves wth a normal appled force of 5 μ and wth dscretaton steps comprsed between 50 nm and μm. Each groove shown s μm n length and orentated at 0 degrees wth respect to the long as of the cantlever. Ths fgure llustrates the devaton of the machned grooves from a lnear lne as a functon of the dscretaton step used wth the Bresenham algorthm. In partcular, as the value of ths step ncreases, the devaton Fg. 5: Illustraton of the Bresenham s lne algorthm from a lnear lne becomes more pronounced and the ndvdual Bresenham segments more observable. The second constrant mposed b the nstrumentaton used was that a tme dela, n the order of a second, takes place between the machnng of each Bresenham segments. More specfcall, ths tme dela occurs ever tme the partcular C++ functon emploed to generate the lateral dsplacement of the stage s called. Ths means that machnng s nterrupted between each segment. Thus, ths nfluences the tme requred to cut a gven pattern dependng on the resoluton of the dscretaton step selected. Consequentl, a compromse should be found between the desred accurac of the machned pattern and the machnng effcenc. The tme taken to machne a segment of μm n length ncludng the mposed dela was measured and based on ths, t was possble to emprcall derve the speed of progresson of the AFM tp, v, n μm.mn - as a functon of the dscretaton step, p, epressed n μm: v = 45. p () Thus, from the epermental results shown n Fg. 6, f a mnmum dscretaton step of 00 nm s chosen for an pattern to be cut, then the mamum cuttng speed achevable n ths case s 4.5 μm.mn -, whch s partcularl slow. However, the deploment of the proposed CAD/CAM approach on dfferent AFM sstems ma not necessarl be lmted b the constrants reported here, and n such cases, hgher cuttng speeds could be acheved. (a) 4 µm Fg. 6: Grooves machned usng the Bresenham algorthm wth dfferent dscretaton steps: (a): AFM mage, (b): SEM mage. The dscretaton steps ndcated on (a) are: groove (): μm, (): 500 nm; (): 00 nm and (4): 50 nm In order to eamne more closel the qualt of the acheved grooves wth dfferent dscretaton steps, another eper- (b) End pont B ment was carred out where 5 μm lnes were cut n the drecton parallel to and towards the cantlever (see Fg. 7). From ths fgure, ple-ups wthn the grooves can be clearl notced when usng a dscretaton step above 00 nm. Ths s due to the nterrupton of the tp progresson between each Bresenham segment. To eplan ths, the load actng on tp s consdered durng both of the successve processng stages. In partcular, the frst stage corresponds to a statc case when the tp s engaged vertcall nto the materal wthout the lateral dsplacement whle the second, dnamc, stage descrbes the lateral progresson of the tp nto the materal accordng to the cuttng drecton shown n Fg. 7. The frst case can be thought of as a nanondentaton operaton when the tp has reached ts mamum penetraton depth durng the loadng ccle. In ths stuaton, the onl force actng on the tp s consdered to be normal to the materal surface and t s referred to as F n. Thus, based on the Euler-Bernoull cantlever beam theor, the vertcal dsplacement, c, of the free end of the probe cantlever can be epressed as follows: c = () EI where L s the length of the cantlever, E s ts modulus of elastct and I s ts second moment of area. In the second case, an addtonal force, F c, actng on the end of the tp and orented horontall s consdered. Ths force s the consequence of the nteracton between the tp and the materal as the probe s moved laterall wth respect to the workpece. Dependng on the machnng condtons, ths nteracton s the result of the partcular processng regme that can take place namel, adherng, ploughng or cuttng. In ths case, the moment, M c, that occurs at the free end of the cantlever and whch s caused b F c actng on the tp must be taken nto account n the epresson of c : c = EI M c + EI 4 () probe tp: Fc. lt c = + (4) EI EI Thus, the vertcal dsplacement, c, of the free end of the cantlever s descrbed b equaton () when the machnng process s nterrupted between the Bresenham segments and b equaton (4) when the tp progresses along such segments. It s mportant to note that, for the frst statc case, a smplfcaton s made b assumng that the force actng on the probe tp has onl a vertcal component, F n. In realt, a horontal force component should also be taken nto account. Ths s due to the fact that, followng the lateral progresson of the probe nto the workpece, the area of contact between the tp and the materal s not smmetrcal wth respect to the as of the tp. However, n the eplanatons reported here, t s assumed that ths load can be neglected n comparson wth the force F c, whch s generated durng the probe lateral dsplacement. It s also mportant to keep n mnd that the feedback loop of the AFM sstem ensures that the vertcal poston, l, of the laser beam reflected from the back of the free end of the cantlever on the photodode s kept constant durng the engagement of the tp wth the materal. It s consdered that l s drectl proportonal to the vertcal movement of the free end of the cantlever, c. Thus, for the frst case, when the machnng operaton s nterrupted, l s formulated as follows: = c.( ) (5) l EI Whle, durng the lateral dsplacement of the probe, l s epressed accordng to equaton (6) below: Fc. lt = c.( + ) (6) l EI EI Therefore, n order to keep l constant throughout machnng and thus, to ensure that the values obtaned from equatons (5) and (6) are equal over tme, the feedback loop of the AFM sstem controls the vertcal poston of the cantlever va the peoelectrc actuator on whch the probe s mounted. In partcular, as the processng condton changes from a statc status (.e. durng the tme spent between Bre- Cuttng drecton L M c c Fg. 7: Effect of the dscretaton step on the qualt of the grooves. The dscretaton steps ndcated are: groove (): μm, groove (): 500 nm; groove (): 00 nm and groove (4): 00 nm Fg. 8 llustrates the loads consdered on the free end of the probe cantlever usng the notatons ntroduced. Equaton () can also be wrtten as follows b defnng l t as the length of the Legend unloaded cantlever deflected cantlever Fg. 8: Schematc of the probe cantlever wth the consdered loads actng at ts free end durng the lateral progresson of the tp nto the materal senham segments) to a dnamc one, the peoelectrc actuator rases the probe n order to decrease the value of F n as a result of the added contrbuton from F c. Conversel, as the status of processng s changed from a dnamc to a statc condton, the probe s lowered n order to ncrease the contrbuton from F n. F n The consequence of successvel rasng and lowerng the probe can also be seen n Fg. 9, whch shows obtaned profles n the cuttng drecton along the bottom of the grooves processed wth Bresenham dscretaton steps of μm (Fg. 9(a)) and 00 nm (Fg. 9(b)). In partcular, regularl spaced ndents wth hgher depths are observed along each groove at postons referred to as, +,, n ths fgure. These ndents correspond to ponts where the processng nterruptons took place and whch are assocated wth a predomnant load contrbuton from F n. The formaton of ple-ups on ether sde of these ndents s clearl vsble n Fg. 9(a). In ths case, a hgher ple-up s formed n front of the tp, whch corresponds to a regon where no pror plastc deformaton took place. In addton, the average depth of the generated ndents reduces as the dscretaton step s decreased. Ths observaton could be due to the fact that, below a gven dscretaton length, the ndents are formed over the ple-up created durng the precedng nanondentaton. In partcular, from the data shown n Fg. 7, t was estmated that the dstance between the bottom of an ndent and the heght of the neghborng ple-ups s between 00 nm and 400 nm. Thus, wth a dscretaton step wthn or below ths range, the depth of the ndent acheved could be affected b stran hardenng of the ple-up materal generated from the precedng nanondentaton operaton. However, further studes, whch are outsde the scope of ths paper, should be conducted to nvestgate ths hpothess. (a) CAD/CAM approach for AFM probe-based nano machnng, a relatvel comple pattern representng half of a snow flake was desgned and subsequentl machned on the brass workpece. The G-cod
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