Layer or Strip Resistance Measurement by Electron Beam Induced Current Technique in a Scanning Electron Microscope

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1 Materials Trasactios, Vol. 48, No. 5 (7). 949 to 953 Secial Issue o New Develomets ad alysis for Fabricatio of Fuctioal Naostructures #7 The Jaa Istitute of Metals Layer or Stri Resistace Measuremet by Electro Beam Iduced Curret Techique i a Scaig Electro Microscoe drzej Czerwiski*, Mariusz Płuska, Jacek Ratajczak, a Szerlig ad Jerzy Kątcki Istitute of Electro Techology, l. Lotikow 32/46, Warsaw, Polad alicatio of scaig electro microscoy (SEM) electro beam iduced curret (EBIC) techique for measuremet of layer or stri resistace ad sheet (surface) resistace is described. I the method a high electro beam curret is used. I this rage the EBIC ( ) deeds o the overall resistace of EBIC circuit. It icludes a secime resistace as well, as geerated carriers create curret that flows alog the measured layer or stri to a ohmic cotact. shift of the electro beam towards the ohmic cotact o the layer (or o the stri) chages resistace of layer (or stri) betwee the e-beam lacemet ad the cotact. The chage of resistace that results is comesated usig a chageable resistace (e.g. a decade resistace box). Provided costat desite the e-beam movemet brigs values of the resistace ad the sheet resistace for the ivestigated layer or stri. Satial distributios of resistace ad local ihomogeeities ca be also revealed. The method was used for the characterizatio of lateral cofiemets i semicoductor laser heterostructures maufactured by Molecular Beam Eitaxy ad wet chemical or reactive io etchig. The method ca be used for very thi layers ad is exected to be alicable for small-size objects as aostructures. [doi: /matertras ] (Received October 27, 6; cceted December 19, 6; Published ril 25, 7) Keywords: scaig electro microscoy, electro beam iduced curret techique (EBIC), semicoductor structure measuremet, resistivity measuremets, semicoductig materials 1. Itroductio ivestigatio by electro beam iduced curret (EBIC) techique of scaig electro microscoy (SEM) is based uo a relatio of icidet electro-beam ositio with EBIC ( ) geerated by the beam i a material. Electro-hole (eh) airs geerated withi the samle by the e-beam diffuse radomly. They would aihilate util all excess carriers were elimiated. However, the carriers that do ot recombie before reachig a regio of sigificat electric field, create. Tyically, it is emloyed as a video sigal, where the sigal-ositio fuctio is visualised to observe ihomogeeities i the electrical roerties of the material (e.g. of coductivity). It ca be alied for various materials, ad tyically is used for semicoductor structures, e.g. for revealig recombiatio sites such as dislocatios ad reciitates. EBIC techique is based o the electric scheme as show i Fig. 1. Overlayers coverig the structure (oxide, metallizatio) are ot show for all schematic drawigs. ohmic cotact at the structure to is marked as the black stri, ad the secod ohmic cotact there is o the structure backside. The electro beam from electro microscoy gu may be cosidered as the curret. Each electro with eergy E beam that loses its whole eergy i the material geerates E beam =E e-h of e-h airs, where E e-h 3:2 E g, 1) is a eergy of e-h air creatio, ad E g is a eergy ga for the material. maximum deth R e of the rage where the geeratio occurs is iversely roortioal to the desity of material ad roortioal to E 1:75 beam, 1,2) or E 1:67 beam,. 3) For tyical semicoductors as Si ad Gas, R e rages from a teth of micrometer at sigle kilovolts to several micrometers at 30 kev. This deth rage ca be ivestigated by EBIC techique. Results of EBIC techique are much ehaced whe the *Corresodig author, aczerwi@ite.waw.l electro beam curret vs beam-ositio deedece is used as raw data for umerical aalyses, i.e. for quatitative EBIC methods. They rovide hysical variables such as the miority carrier lifetime, diffusio legth ad surface recombiatio i semicoductor materials. 1,2,4 8) ssumig from exerimets or theory the maximum EBIC sigal for ivestigated material, overlayer thickesses that weakeed the sigal ca be calculated. 1,9) Desite strog geeral relatio of EBIC techique with a electric coductivity, it has bee ractically uused for measurig a resistace of layers or stris. The oly EBIC aroach to the determiig of layer resistace reseted u to date was based o tedious fittig of shaes of exerimetal satial distributios by characteristics calculated from comuter simulatios, with guessig the sheet (surface) resistace (R S ) of layer. 10) Beside that it was a fittig (ot a measuremet) aroach, its tyical results were urealistic. It eglects: (i) the lack of lateral flow of carriers geerated i or flowig through the deletio regio (due to large lateral resistace withi this regio, excet of a small bulb where the geeratio occurs), ad (ii) the diffusiolegth related limitatio of the lateral flow of carriers Fig. 1 Schematic drawigs (ot i scale) showig the EBIC techique for a examle semicoductor structure. The icidet electro-beam creates sigal from the viciity of e-beam ositio.

2 950. Czerwiski, M. Płuska, J. Ratajczak,. Szerlig ad J. Kątcki geerated outside the deletio regio before they reach the layer. The objective of reset work is to show the first time a ractical method utilizig EBIC techique for the measuremet of resistaces ad sheet resistaces, satially uiform or ouiform, of layers or stris. 2. Exerimetal Procedure The method utilizes a rage of e-beam curret ( ) sufficietly high i relatio to the overall resistace of the EBIC circuit (R EBIC ). It is equivalet to the rage of sufficietly large R EBIC i relatio to used i the exerimet. For such ad R EBIC, = ratio decreases whe R EBIC is icreased. It is differet from tyical quatitative-ebic alicatios with low currets ad costat = ratio. For them the ratio deeds o the desity ad thickess of overlayers that weake a e-beam, ad o the material where the geeratio occurs, but ot o resistaces i the EBIC circuit. Geerally, whe curret icreases at costat both E beam ad R EBIC (Fig. 2(a)), the curret icreases roortioally (with costat = ratio) util a secific. For higher, = ratio decreases, iitially slowly, while for high values becomes almost costat. O the other had, whe R EBIC resistace icreases at costat both E beam ad (Fig. 2(b)), the (i.e. also = ratio) remais ractically costat util a secific R EBIC. For higher R EBIC, decreases. This decreasig is iitially faster, while for large R EBIC values the R EBIC roduct (i.e. the dro of otetial) becomes almost costat, deedet o. Each high-eergy electro geerates E beam =E e-h of e-h airs i the ivestigated material. However, for high currets all geerated carriers caot form curret, because i EBIC circuit it would create a large dro of otetial, equal to the roduct R EBIC. Therefore, i this rage, if a overall resistace comoet icreases, the decreases ad excess electro-hole (e-h) airs aihilate due to icreased recombiatio. The deedece of measured at small R EBIC (i.e. the maximum at secific ) o the roduct R EBIC measured at the same ad large R EBIC, resembles the forward curret vs voltage (I V) characteristic of the juctio resosible for EBIC collectio. It meas that while for low (or small R EBIC ) a EBIC heomeo serves as a curret source i EBIC electrical circuit, for high (or large R EBIC ) this heomeo creates a voltage source, able to force a secific voltage dro i EBIC circuit. To measure the resistace ad the sheet resistace by the method, it is cosidered that curret geerated withi or i the viciity of ivestigated layer or stri flows through it. The e-beam is shifted towards the ohmic cotact o this layer or stri (Fig. 3). Therefore, deedig o the lacemet of e- beam, flows towards the ohmic cotact through a larger or smaller resistace. Resistaces of other arts of EBIC circuit are ideedet from the e-beam ositio, e.g. flows i other arts of secime eredicularly to the juctio lae towards a ohmic cotact o the structure backside, as i Fig. 3. Curret, / µ Curret, / µ (a) (b) / = R EBIC = 0 R EBIC = 0 Ω 2 kω 3 kω 5 kω 10 kω 50 kω Curret, / = / I beam = R EBIC = Resistace, R EBIC / kω Fig. 2 The exerimetal deedeces of the EBIC curret for the ivestigated semicoductor structure (at the mesa regio): (a) deedece o the e-beam curret, measured at various costat overall resistaces R EBIC of EBIC circuit, ad (b) deedece o R EBIC, measured at various. The beam eergy E beam ¼ 30 kev ad = ¼ 880 at low rage. Whe a exteral chageable resistace (e.g. decade resistace box) is icluded ito the circuit (Fig. 3), it allows the comesatio of resistace chages caused by the e-beam shift. Because a icrease of the total resistace R EBIC is resosible for the decrease of, the imosed coditio of the costat durig measuremets makes R EBIC costat desite the e-beam shift. bsolute values of subsequet chages that kee costat durig e-beam shift are equal to chages of layer or stri resistace durig the shift. The measured resistace is give by a differece ðd ¼ 0Þ ðdþ betwee the comesatio resistaces at two e-beam ositios, where d 0 is the distace of e-beam from its iitial ositio (Fig. 3). Whe a icreasig of resistace with the e-beam distace becomes liear, the sheet resistace R S for the ivestigated layer or stri ca be revealed from the sloe of a related lot where

3 Layer or Stri Resistace Measuremet by Electro Beam Iduced Curret Techique i a Scaig Electro Microscoe 951 (a) (b) electro beam shift electro beam shift + the distace is trasformed to the umber of squares d=w, where W is the layer or stri width. Philis XL30 scaig electro microscoe with LaB 6 electro gu was imlemeted. The scheme of measuremet set u is show for the cases of a examle resistive layer (Fig. 3(a)) ad a examle resistive stri (Fig. 3(b)). Tyes of doig ( or ) ca be exchaged. Isolatio ad coductig overlayers, which cover the structure, are ot show. The method was alied for characterizig buried layers that costitute lateral cofiemets i the lgas/gas laser heterostructures with 8 m thick IGas quatum well, maufactured with Molecular Beam Eitaxy (MBE) (Fig. 4). Layers above semicoductors cosist of oxide ad metallizatio, with ohmic cotacts to þ -Gas o the mesa to ad to þ -Gas o the structure backside. The structures are examles of the ridge waveguide laser, which is the most tyical semicoductor laser. It has lateral cofiemets o both mesa sides, which defie a arrow lasig regio uder + Fig. 3 Schematic drawigs (ot i scale) showig resistace measuremets with the method: (a) for a examle layer, ad (b) for a examle stri. The sigal, evertheless a shift of icidet electro-beam that chages its distace from the ohmic cotact, is ket costat by usig for the comesatio a chageable resistor. electro beam shift + + Gas lgas lgas + Gas Fig. 4 Schematic drawigs (ot i scale) showig the resistace measuremet with the method for -tye lateral cofiemets i lgas/gas ridge waveguide laser heterostructure with IGas quatum well. mesa. overflow of laser curret outside the regio uder mesa is miimized by large resistaces of lateral cofiemets, as -lgas claddig is thied there by a etchig rocess. Wet chemical ad reactive io etchigs (RIE) were alied. For better exlaiig the method, was measured usig the Faraday cu laced besides the ivestigated structure i the microscoe chamber. Such measuremet of is uecessary for the roosed method itself. The relative chages of the -claddig layer thickess i the regio of lateral cofiemets were determied from SEM images of the structure cross-sectios, take i secodaryelectro SEM mode. The lateral cofiemets i structures uderwet electrobeam ijectio i SEM with the e-beam shifted eredicularly to the mesa edge (Fig. 4). 3. Results For high values, icreases as the e-beam is shifted towards the ohmic cotact at mesa. Satial chages of are almost idetical o both sides of mesa for the ioetched lateral cofiemets (Fig. 5(a)), while for the wetetched oes differeces occur betwee satial chages o oosite sides of the mesa (Fig. 5(b)). Such a big differece of sigal ca be also visible i EBIC image (Fig. 5(c)). fter iitial, ofte o-liear chages of the resistace for low umber of squares d=w (where d ¼ 0 corresods here to the mesa edge), a icrease of resistace becomes liear for larger distaces from mesa. The lots of resistace versus umber of layer squares are show i Fig. 6. The sloes of liear arts give similar R S values (from 4900 to 5 / square) for three io-etched samles, while much disersed R S values (from 2950 to 7600 /square) for three wet etched samles. Small values of L=W above which the liearity is achieved are cosistet with a similar case of a curret flowig alog resistive stris, 11) where a imact of a cotactad lacemet o the overall resistace of stri is cosidered. R S resistace deeds o both MBE ad etchig rocesses. Relative chages of lateral cofiemet thickess for wetetched lasers maufactured i the same MBE rocess were determied usig SEM secodary-electro mode observatios of each structure cross-sectio. The thickess chages were comared i Fig. 7 with R S values revealed for these samles. It is visible that the revealed R S values decrease for thicker lateral cofiemets, as exected. 4. Discussio The method is alicable for all structures where EBIC techique ca be used. The measured layer or stri should be ot short circuited e.g. by a Schottky cotact o its uer side. The resistace should (i) assure that R EBIC is i the rage where is deedet o R EBIC at the alied ad E beam, ad (ii) eable a accurate comesatio of resistace chages durig the e-beam shift. The liearity of lot is disturbed i the viciity of mesa. Due to diffusio the carriers geerated at oe lace reach the juctio deletio regio ot oly at this e-beam sot but also

4 952. Czerwiski, M. Płuska, J. Ratajczak,. Szerlig ad J. Kątcki Fig. 6 Plots of resistace vs umber of squares for thi -lgas lateral cofiemets i lgas/gas laser heterostructures. The sloes of liear arts give similar R S values (from 4900 to 5 /square) for three io etched samles, while R S values are much disersed (from 2950 to 7600 /square) for three wet etched samles Wet etched structures from oe MBE rocess Resistace, R S / Ω Fig. 5 Differeces of EBIC sigal ( = ratio) due to differet resistaces of lateral cofiemets i lgas/gas laser heterostructure for cosecutive ositios of e-beam alog a lie eredicular to the mesa edge for (a) io reactive etchig ad (b) wet chemical etchig. I both cases the e-beam eergy E beam ¼ 30 kev, the e-beam curret ¼ 125, ad the iitial comesatig resistace ¼ 7 k. big differece of EBIC sigals from both lateral cofiemets i (b) is visible i the corresodig EBIC image (c). at distaces closer or further to the ohmic cotact at mesa. Each grou of carriers that reaches the juctio further from the mesa has its couterart reachig the juctio equally closer to the mesa. Both chages of the distace cacel themselves ad the average distace is equal to that of the geeratio sot. However, whe the geeratio occurs close to the mesa, the carriers reachig the juctio further ad further from the mesa have all couterarts reachig it at mesa. Therefore a average legth of flow icreases above the distace betwee the geeratio sot ad the mesa, Differece of layer thickess / m Fig. 7 Relative chages of R S values for wet etched lgas/gas laser heterostructures maufactured all i the same MBE rocess, comared with lateral-cofiemet thickesses revealed for these samles. elargig measured there. The magitude of this effect deeds o the diffusio legths of miority carriers. stroger local recombiatio may ifluece the liearity of the resistace versus distace lot i ay lace. Whe less carriers reach the juctio, measured is lower, similarly as for larger R S. If a icreased defectivity aears oly i distict sots, accurate measuremets may be erformed, if oly omittig measuremets i their viciity. lso the surface recombiatio may weake a EBIC sigal whe the e-beam is laced earby outer edges of the whole laser structure. The liearity of the resistace vs distace lot, as visible i Fig. 6, cofirms a costat recombiatio i the measuremets.

5 Layer or Stri Resistace Measuremet by Electro Beam Iduced Curret Techique i a Scaig Electro Microscoe 953 Due to high satial resolutio eabled by SEM, the method eables resistace measuremets of very small elemets ad it is sesitive to ouiformities i resistace distributio. Such distributio ca hae due to techological reasos, e.g. whe a resistive stri behaves as a series of resistors with differet resistivity. 12) ouiform distributio of resistace ca be distiguished from a ouiform distributio of recombiatio by alyig various E beam or various iitial ðd ¼ 0Þ. Deedig o E beam the geeratio may occur at differet deths i the juctio: i the - ad -tyes of quasieutral regios (where the carrier diffusio takes lace), ad i the deletio regio (where the carriers are swet off ad their lateral flow is egligible). icrease of E beam moves the geeratio regio deeer. It chages a roortio of geeratio i differet regios as well as a average imact of recombiatio (ad of its ouiformity) o results. licatio of differet E beam allows also for the lacemet of geeratio regio at deth where this ouiformity is miimal. O the other had, a icrease of R EBIC icreases the umber of aihilated carriers, hece also a imact of recombiatio. If the comesatio resistace ðdþ shows the same shae of oliear distributio desite sigificat chages of E beam or of iitial ðd ¼ 0Þ, it cofirms that this oliearity ca be attributed to a ouiform distributio of sheet resistace. Whe the above reeatability of lots cofirms ouiformities i R S distributio, it is esecially iformative for arrow stris, where it reresets the local values, while for wide layers it gives effective values deedig o the whole layer. Very thi layers ca be measured with the method, e.g. havig EBIC sigals as visible i Figs. 5(b) ad 5(c) o the right. EBIC techique ca be alied for aostructures, e.g. EBIC images from delta-doed Gas quatum wires, 1 mm wide, have bee roduced. 13) Deedig o SEM istrumet, the e-beam sot size reaches 1 2 m or eve 0.5 m, 14) although for higher currets it icreases. The e-beam shift is also available i the rage of aometers. Moreover, it is ossible to ositio the deth of electro beam so that carrier geeratio occurs maily withi the deletio layer. 1) Uder these circumstaces, electros ad holes drift i oosite directios ad afterwards joi the quasi-eutral materials as majority carriers. The lateral diffusio of carriers that lowers EBIC satial resolutio is omitted i these coditios. Thus, a good satial resolutio, sufficiet eve for the ivestigatio of aostructures with this method, ca be achieved. 5. Coclusios ovel method of EBIC alicatio is reseted, which for the first time eables determiig the resistace ad the sheet resistace (surface resistace), satially uiform or ouiform, for layers ad stris, tyically i semicoductor structures. Because ay kowledge of theoretical or exerimetal coefficiets ad distributios is uecessary i the roosed method, it is less comlicated tha may other quatitative EBIC aroaches. The method eables also a characterizatio of buried resistive regios covered by other layers. Not oly the sheet resistace but also its satial distributio ca be revealed with high resolutio. The method ca be also used for very thi layers ad is exected to be alicable for a wide rage of object sizes, from thick ad thi layers, to arrow stris ad aostructures. REFERENCES 1) H. J. Leamy: J. l. Phys. 53 (1982) R51 R80. 2) D. K. Schroder: Semicoductor Material ad Device Characterizatio, Secod Editio (Wiley-Itersciece, New York, 1998) ) K. Kaaya ad S. Okayama: J. Phys. D 5 (1972) ) C. Doolato: l. Phys. Lett. 43 (1983) ) C. Doolato: l. Phys. Lett. 46 (1985) ) R. Scheer, M. Wilhelm ad L. Stolt: l. Phys. Lett. 70 (1997) ) S. Hildebradt, J. Schreiber, W. Hergert, H. Uiewski ad H. S. Leier: Scaig Microscoy 12 (1998) ) M. Gaevski, M. Elfwig, E. Olsso ad. Kvist: J. l. Phys. 91 (2) ). Czerwiski, M. Płuska, J. Ratajczak ad J. Kątcki: Joural of Microscoy 224 (6) ) J. Rechid ad R. Reieke-Koch: Thi-Solid-Films 387 (1) ) D. Tomaszewski: Proc. Istitute of Electro Techology (i Polish), issue 1, (1981) ) F. La Via, S. Privitera, M. G. Grimaldi, E. Rimii, S. Quilici ad F. Meiardi: Microelectroic-Egieerig 50 (0) ) D. J. C. O Neill, Y. Feg, T. J. Thorto ad J. J. Harris: Proceedigs of Microscoy of Semicoductig Materials: 8th Oxford Coferece (SMM VIII). Oxford, UK, , IOP, Bristol, UK (1993). 14) J. C. Gozalez, M. I. N. da Silva, K. L. Buker,. D. Batchelor ad P. E. Russell: Microelectroics Joural 34 (3)