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Ellipsometry, low-energy electron diffraction and field electron microscopy combined
SURFACE SCIENCE 9 (1968) 476-483 © North-Holland Publishing Co., Amsterdam
ELLIPSOMETRY, LOW-ENERGY ELECTRON DIFFRACTION AND FIELD ELECTRON MICROSCOPY COMBINED l~:eceived I1 August 1967
An experiraental approach has been developed which simultaneously applie:: three techniques to the study of surface phenomena. This approach was designed to exploit the complementary features of the techniques a,, well as to corapare the measurements of each where they provide similar information. The interpretation of experimental data is thereby significantly simplified by the removal of uncertainties which often exist due to differences in experimental conditions when several separate experiments are compared. The observational techniques that are included in this experimental apparatus are: Ellipsometry rE), Low-energy electron diffraction (L) and Field-electron emission microscopy (F). The essential features of the composite ELF instrument are shown schematically in fig. 1. Ellipsometry gives information about the amount of material absorbed on a surf~ce and its i,.adex of" refraction by measuring the relative phase retardati~, A, and the relative amplitude reduction tan ~,, that occurs when polarized ~!Qht is reflected from the surface. Methods for the interpretation of elliF,sometric data in studies of thin films are well known ~). In the ELF, the quartz entrance and exit wihdows are positioned for an angle of incidence of 70 °, which gives good sensitivity ~). Low-energy electron diffraction gives information concerning the config,rationt of,the outermost atomic layers by analyzing the spatial distribution of the back-scattered electrons :~). The electrode arrangement used in the ELF is typical of the apparatus generally used for LEED studies d); a flat grid sy~,tem was preferred to th: _eherical configuration to facilitate the entrance arKd exit of the light used by the ellipsometer. Field-electron emission ~nicroscopy pr~,vides information about the electronic work function aF~d the geometry of the surface 5) by analysis of the flow and spatial di'.tribution of electrons field-emitted from the specimen under study6). In contrast to the above twt~ techniques, which measure the average propeT"ties of an area of macroscopic size, the field emission microscope provices a highly magnified image of a crystal surface. A field emitter, cut fr~,m the same crystal in the same orientation as the primary .';ample (S in fig. 1), is located about 2 cm away from S. (In 476
ELLIPSOMETR¥,
LEED
AND
477
FEM
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I I ,~"----PHOSPHOR SCREEN
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ELECTRON GU~ -
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\ Fig. I. Schematic drawing of main elements of ELF tube. E indicates light beam for eilipsometry on primary sample S. L indicates diffracted low-energy electron beam. F indicates field-electron emitter assembly pointing at sectior of phosphor screen. W indicates quartz windows.
some experiments it was spot welded directly to S.) It is used for vacuum mo~aitoring in the immediate neighborhood of the primary sample and for providing FEM information ,luring adsorption and desorption expe~'iments. The composite tube is attached to a bakeable metal, sputter ion pumped vacuum system which is capable of maintaining a pressure of approxim~ teiy 10-1o Torr during operation. Pre,,isions exist for the controlled introduction of severM gases and for spectrometric mass analysis of the rcsidua! gas content. At pre~ent, the ellipso;neter is used simultaneously with either the LEED or the field elnitter, as the very different potenti~tl and ieid conditions preclude the~_r use tog..'ther; pulse techniques, however, couht be used to remove this limitation ant~ also would permit meaningful contin~ous field emission observation during adsorption.
478
.,~. J. MEII.MED, H. P. I.AYER A N D J. K R U G E R
~o initiate the investigation of the capabilities of the E L F instrument and the relative sensitivities of the three techniques involved, the adsorption of ox>gen on ,~arious l:aees of tungsten is being studied. This system was chosen because tungsten is considered to be an easy material to clean and because a considerabte amount of infoqnation about the system exists in the literature. The experimental sequence was as follows" I. The sa: ~p~es (primary sample S and field emitter) were cleaned thermally and the sutfac~." " -s characterized by each technique. 2. The samples acre exposed to oxygen and then characterized after each exposure. 3. The samples were heated to 1000':C for I00 sec at 10-~o T o r t (ref. 7). Some results of this experimental sequence are shown in table I and fig. 2. As fig. 2 shows it is difficult to detect any apparent changes between the TABI.T 1 C h a n g e s detected by ellipsometer
Conditions
Oxygen at 5
10 r T c r r
Refractive index Real part Imaginary part
Changes in/J
Chan%es in ~,
Thickness
¢).3~{
0.02 ~
Ranging from I~1 ~o 3,5 ,~
Ranging fcom ~.~ o 1.5
Ranging from 0.~ ~ ~.0
t?.5.~
~.~
2.3 ov 2.8 ,~
!.9 ~w 2.4
2,5 ~,~ 2,n
Oxygen aI 5 . ~0 rTo~.r K~r 240 scc and ~hcn hca~cd ~'or 100 see at
"~cIean L E E D pattern", (a), and that obtained from a surface exposed to the indicated dose of oxygen*, (b), while the VEM pattern For ~he same conditions (d} and (e) are markedly different. As ~able ~ shows the ellipsometer was able to detect di~erences between the c~ean surface and o n e exposed ~o oxygen, with A~ the most sen',itive parameter, changing by 0 . 3 8 . The values of thickness and refractive index in taNe ~ we~e calct~ated from ~he experimen~a! determi~ations <~' changes in A and ~!~ using the exac~ Drude equa~ic, n~v). The caicu~a~ions for the very ~hin ~itm ~)rmed during s~ep 2 of ~he expcHmen~.a~ sequence gave a number of v~t~es for ~hicknc%s and re{"~c~i~e inde>;~ each ~>{wi~i~ch ~i{ted {he measurem,en~s cqt~a~y well. These arc a monotayer and are g~ven as a range ofva~m.'s in mb~e ~. A bettec approach may be {ha~ given by Archer ~o). ~.Jpon reconstruction {s~:ep 3) ~}nIy {we se~ts :~ 7hi~-~ dc.~c~ r ~ preclude ~hc ~cc~r.re~-~ce of changes in ~he funclionaU dependence oH" s O ~ in~cn~i~ies '~s. electron w a v e k n ~ h ~ } .
Fig. 2a
Fig. 2b
Fig. 2.
LEED and FEM patterns.
a, b, c: L~!iED pm¢cms ai ~60 V from ~00l)-oriented W. (a) Clean W ; (b) Same sample a~lcr e x p ~ . u r e to ~.2 ~: 10 ~ Torr-sec oxygen at r o o m ~emperalure; (c) S a ~ e sample after healing a~ 1000 C for 100 scc at tO TM Tc, rr.
d, c, f: YIIiM pa~tcrns from (OO~)-orien~:ed W lip treated similarly ~o sample of a, b, ~:. (d) ( k'a~ W, 42~0 V; (e) After r o o m ~em~erature oxygen exposure, 7501~ V; ~f~ After hea~ treatmen: a~ lO00 C, 7~00 V.
4£~}
A. ,~. M} I MI-]),, H . P. I AY[!R A N [ ) .|. KRU(;~ER
Fig. 2c
l:::ig. 2d
|!I_I.IPS()METRY, ILr~Ll) AND t:EM
4~]
Fig. 2e
Fig. 2i
482
A . J . MF.LMED~ H. P, LAYER ANt.) $. KRUGER
of values were obtained from the calculations. These i:adicated an increase in the refractive index, a reasonable expectation if recenstruction results in a film containing both tungsten and oxygen at0ms. The LEED pattern (fig. 2c) also indicates this result¢). FEM: (fig, 2e)~indiea~s ¢0n~derable change in the (00~) r e , on but does not by itself give infomafioJl about the arrangement of the tungsten and oxygen atoms, Considerable care must be exercised in making quantitative comparisons of observations and measurements on a flat siLgle c~stal surface (S) and on a curved single crystal surface (field emitter), Such detailed comparison is outside the intended scope of this report. With regard to relative sensitivities, preliminary qualitative observations indicated that the field-electron emission microscope displayed the greatest sensitivity for detecting the !first stages of adsorption. This result was found during the initial stages of oxygen adsorption and during adsorption of ~esidual gases (mostly hydrogen and CO) in the ELF tube. Also, during the initial specimen cleaning process, LEED showed patternswhich in the literature are considered representative of "clean" surfaces while the FEM patterns clearly indicated that adsorption was occurring. Further heating of the sample and improvement of the vacuum ~.ncreased the stability of the FEM pattern and produced subtle changes (increased signal-noise ratio) in the' LEED pattern. This indicates that some caution should be exercised in using a particular LEED pattern as a criterion for cleanliness of surfaces. Quantitative measurements by the three techniques will be necessary to establish ~:he comparative sensitivities. Using the electron emission pavtern from the entiire field-emitler surfaces as in the present work, impingement rates (on the: primary sample as wall as the emitter) could be estimated by observing changes in the over-all pattern. However, on planes ~vhich are aormally dark, as is t~he case with (001) tungsten, the sensitivity of the FEM m/~y not be better than LEED or ellipsometry. The ultimate comparison of sensitivities of the three techniques must be made (:['or a given a.,IsorptiGn system) using each technique in its most sensitive, quantitative mann~ r ~, a, :~.,8).
Acknowledgement The auth~rs wislh to thank the United States De~art:--r..ent of the !nterior, Otfice of Saline Water. for supportine ~ ~uo~ar~t~l~lpart of this work.
?detallurgy Division, l,;~,~;it:erejot Materials Research, National Bureau of Sgandards, ~/ash?Tgzoir, O.C. 20234, U.& A.
A. J. MELMED, H. P. L:..¥ER and J. KRL~OER
ELLll~OMETRY, LEED AND FEM
483
References 1) See for example, Ellipsometry in the Measurement of Surfaces and Thin Films, Symposium P r ~ M ~ i ~ Washington, 1963, Eds. E. Passaglia, R. R: Stromberg and J. Kruger, U.S~ ~ a ~ m e ~ t Of Commerce, Natl. Bur. Stcl, Misc. PubL 256 (1964). 2) Rmhard C:~Smith:and~Michaei H~sk~Yl0, ibid. 3) J. W. May, Ind; Eng. Chem.: July(1965)18. 4) J. J. Lander, F. C. Umerwald and S. Morri~on, Rev. Sci. Instr. 33 (1962) 784. 5) R. Gomer, Field Emission aM Fieldlonization (Harvard Univ. Press, Cambridge, Mass., 1961). 6) R. H. Good, Jr., and Erwin W. MiiI!er, in: Handbuch tier Physik, Band 21, Ed. S. Fliigg.~ (Springer-Verlag, Berlin, 1956) p. 176. 7) J. Anderson and W. E. Danforth, J. Franklin Inst. 279 (1965) 160. 8) g. L. Park and H. E. Farnsworth, Surface Sci. 2 (1964) 527. 9) A. B. Wintorbottot~, Optical Studies of Metal Surfaces, Kgl. Norske Videnskab. Selskabs Skrifter (I955) 1 (F. Bruns Bokhandel, Trondheira, 1955L 10) R. J. Archer, ref. 1, p. 255
ELLIPSOMETRY, LOW-ENERGY ELECTRON DIFFRACTION AND FIELD ELECTRON MICROSCOPY COMBINED l~:eceived I1 August 1967
An experiraental approach has been developed which simultaneously applie:: three techniques to the study of surface phenomena. This approach was designed to exploit the complementary features of the techniques a,, well as to corapare the measurements of each where they provide similar information. The interpretation of experimental data is thereby significantly simplified by the removal of uncertainties which often exist due to differences in experimental conditions when several separate experiments are compared. The observational techniques that are included in this experimental apparatus are: Ellipsometry rE), Low-energy electron diffraction (L) and Field-electron emission microscopy (F). The essential features of the composite ELF instrument are shown schematically in fig. 1. Ellipsometry gives information about the amount of material absorbed on a surf~ce and its i,.adex of" refraction by measuring the relative phase retardati~, A, and the relative amplitude reduction tan ~,, that occurs when polarized ~!Qht is reflected from the surface. Methods for the interpretation of elliF,sometric data in studies of thin films are well known ~). In the ELF, the quartz entrance and exit wihdows are positioned for an angle of incidence of 70 °, which gives good sensitivity ~). Low-energy electron diffraction gives information concerning the config,rationt of,the outermost atomic layers by analyzing the spatial distribution of the back-scattered electrons :~). The electrode arrangement used in the ELF is typical of the apparatus generally used for LEED studies d); a flat grid sy~,tem was preferred to th: _eherical configuration to facilitate the entrance arKd exit of the light used by the ellipsometer. Field-electron emission ~nicroscopy pr~,vides information about the electronic work function aF~d the geometry of the surface 5) by analysis of the flow and spatial di'.tribution of electrons field-emitted from the specimen under study6). In contrast to the above twt~ techniques, which measure the average propeT"ties of an area of macroscopic size, the field emission microscope provices a highly magnified image of a crystal surface. A field emitter, cut fr~,m the same crystal in the same orientation as the primary .';ample (S in fig. 1), is located about 2 cm away from S. (In 476
ELLIPSOMETR¥,
LEED
AND
477
FEM
w
i
E
i
/
"~.
/
/
.'-
/
./,.h" S-"-~IE~ ' . ~_..,.x',\
/
I I ,~"----PHOSPHOR SCREEN
/ ill
.
.
.
m
\"X ~ i
,,'~', T ' ~
.,.,--~ll x
~
\
\
ELECTRON GU~ -
II II
~
\\
1 !"
-
~\ \ 1 I
~~ETARD !
SCREEN
N-
-
Ik ____
\ Fig. I. Schematic drawing of main elements of ELF tube. E indicates light beam for eilipsometry on primary sample S. L indicates diffracted low-energy electron beam. F indicates field-electron emitter assembly pointing at sectior of phosphor screen. W indicates quartz windows.
some experiments it was spot welded directly to S.) It is used for vacuum mo~aitoring in the immediate neighborhood of the primary sample and for providing FEM information ,luring adsorption and desorption expe~'iments. The composite tube is attached to a bakeable metal, sputter ion pumped vacuum system which is capable of maintaining a pressure of approxim~ teiy 10-1o Torr during operation. Pre,,isions exist for the controlled introduction of severM gases and for spectrometric mass analysis of the rcsidua! gas content. At pre~ent, the ellipso;neter is used simultaneously with either the LEED or the field elnitter, as the very different potenti~tl and ieid conditions preclude the~_r use tog..'ther; pulse techniques, however, couht be used to remove this limitation ant~ also would permit meaningful contin~ous field emission observation during adsorption.
478
.,~. J. MEII.MED, H. P. I.AYER A N D J. K R U G E R
~o initiate the investigation of the capabilities of the E L F instrument and the relative sensitivities of the three techniques involved, the adsorption of ox>gen on ,~arious l:aees of tungsten is being studied. This system was chosen because tungsten is considered to be an easy material to clean and because a considerabte amount of infoqnation about the system exists in the literature. The experimental sequence was as follows" I. The sa: ~p~es (primary sample S and field emitter) were cleaned thermally and the sutfac~." " -s characterized by each technique. 2. The samples acre exposed to oxygen and then characterized after each exposure. 3. The samples were heated to 1000':C for I00 sec at 10-~o T o r t (ref. 7). Some results of this experimental sequence are shown in table I and fig. 2. As fig. 2 shows it is difficult to detect any apparent changes between the TABI.T 1 C h a n g e s detected by ellipsometer
Conditions
Oxygen at 5
10 r T c r r
Refractive index Real part Imaginary part
Changes in/J
Chan%es in ~,
Thickness
¢).3~{
0.02 ~
Ranging from I~1 ~o 3,5 ,~
Ranging fcom ~.~ o 1.5
Ranging from 0.~ ~ ~.0
t?.5.~
~.~
2.3 ov 2.8 ,~
!.9 ~w 2.4
2,5 ~,~ 2,n
Oxygen aI 5 . ~0 rTo~.r K~r 240 scc and ~hcn hca~cd ~'or 100 see at
"~cIean L E E D pattern", (a), and that obtained from a surface exposed to the indicated dose of oxygen*, (b), while the VEM pattern For ~he same conditions (d} and (e) are markedly different. As ~able ~ shows the ellipsometer was able to detect di~erences between the c~ean surface and o n e exposed ~o oxygen, with A~ the most sen',itive parameter, changing by 0 . 3 8 . The values of thickness and refractive index in taNe ~ we~e calct~ated from ~he experimen~a! determi~ations <~' changes in A and ~!~ using the exac~ Drude equa~ic, n~v). The caicu~a~ions for the very ~hin ~itm ~)rmed during s~ep 2 of ~he expcHmen~.a~ sequence gave a number of v~t~es for ~hicknc%s and re{"~c~i~e inde>;~ each ~>{wi~i~ch ~i{ted {he measurem,en~s cqt~a~y well. These arc a monotayer and are g~ven as a range ofva~m.'s in mb~e ~. A bettec approach may be {ha~ given by Archer ~o). ~.Jpon reconstruction {s~:ep 3) ~}nIy {we se~ts :~ 7hi~-~ dc.~c~ r ~ preclude ~hc ~cc~r.re~-~ce of changes in ~he funclionaU dependence oH" s O ~ in~cn~i~ies '~s. electron w a v e k n ~ h ~ } .
Fig. 2a
Fig. 2b
Fig. 2.
LEED and FEM patterns.
a, b, c: L~!iED pm¢cms ai ~60 V from ~00l)-oriented W. (a) Clean W ; (b) Same sample a~lcr e x p ~ . u r e to ~.2 ~: 10 ~ Torr-sec oxygen at r o o m ~emperalure; (c) S a ~ e sample after healing a~ 1000 C for 100 scc at tO TM Tc, rr.
d, c, f: YIIiM pa~tcrns from (OO~)-orien~:ed W lip treated similarly ~o sample of a, b, ~:. (d) ( k'a~ W, 42~0 V; (e) After r o o m ~em~erature oxygen exposure, 7501~ V; ~f~ After hea~ treatmen: a~ lO00 C, 7~00 V.
4£~}
A. ,~. M} I MI-]),, H . P. I AY[!R A N [ ) .|. KRU(;~ER
Fig. 2c
l:::ig. 2d
|!I_I.IPS()METRY, ILr~Ll) AND t:EM
4~]
Fig. 2e
Fig. 2i
482
A . J . MF.LMED~ H. P, LAYER ANt.) $. KRUGER
of values were obtained from the calculations. These i:adicated an increase in the refractive index, a reasonable expectation if recenstruction results in a film containing both tungsten and oxygen at0ms. The LEED pattern (fig. 2c) also indicates this result¢). FEM: (fig, 2e)~indiea~s ¢0n~derable change in the (00~) r e , on but does not by itself give infomafioJl about the arrangement of the tungsten and oxygen atoms, Considerable care must be exercised in making quantitative comparisons of observations and measurements on a flat siLgle c~stal surface (S) and on a curved single crystal surface (field emitter), Such detailed comparison is outside the intended scope of this report. With regard to relative sensitivities, preliminary qualitative observations indicated that the field-electron emission microscope displayed the greatest sensitivity for detecting the !first stages of adsorption. This result was found during the initial stages of oxygen adsorption and during adsorption of ~esidual gases (mostly hydrogen and CO) in the ELF tube. Also, during the initial specimen cleaning process, LEED showed patternswhich in the literature are considered representative of "clean" surfaces while the FEM patterns clearly indicated that adsorption was occurring. Further heating of the sample and improvement of the vacuum ~.ncreased the stability of the FEM pattern and produced subtle changes (increased signal-noise ratio) in the' LEED pattern. This indicates that some caution should be exercised in using a particular LEED pattern as a criterion for cleanliness of surfaces. Quantitative measurements by the three techniques will be necessary to establish ~:he comparative sensitivities. Using the electron emission pavtern from the entiire field-emitler surfaces as in the present work, impingement rates (on the: primary sample as wall as the emitter) could be estimated by observing changes in the over-all pattern. However, on planes ~vhich are aormally dark, as is t~he case with (001) tungsten, the sensitivity of the FEM m/~y not be better than LEED or ellipsometry. The ultimate comparison of sensitivities of the three techniques must be made (:['or a given a.,IsorptiGn system) using each technique in its most sensitive, quantitative mann~ r ~, a, :~.,8).
Acknowledgement The auth~rs wislh to thank the United States De~art:--r..ent of the !nterior, Otfice of Saline Water. for supportine ~ ~uo~ar~t~l~lpart of this work.
?detallurgy Division, l,;~,~;it:erejot Materials Research, National Bureau of Sgandards, ~/ash?Tgzoir, O.C. 20234, U.& A.
A. J. MELMED, H. P. L:..¥ER and J. KRL~OER
ELLll~OMETRY, LEED AND FEM
483
References 1) See for example, Ellipsometry in the Measurement of Surfaces and Thin Films, Symposium P r ~ M ~ i ~ Washington, 1963, Eds. E. Passaglia, R. R: Stromberg and J. Kruger, U.S~ ~ a ~ m e ~ t Of Commerce, Natl. Bur. Stcl, Misc. PubL 256 (1964). 2) Rmhard C:~Smith:and~Michaei H~sk~Yl0, ibid. 3) J. W. May, Ind; Eng. Chem.: July(1965)18. 4) J. J. Lander, F. C. Umerwald and S. Morri~on, Rev. Sci. Instr. 33 (1962) 784. 5) R. Gomer, Field Emission aM Fieldlonization (Harvard Univ. Press, Cambridge, Mass., 1961). 6) R. H. Good, Jr., and Erwin W. MiiI!er, in: Handbuch tier Physik, Band 21, Ed. S. Fliigg.~ (Springer-Verlag, Berlin, 1956) p. 176. 7) J. Anderson and W. E. Danforth, J. Franklin Inst. 279 (1965) 160. 8) g. L. Park and H. E. Farnsworth, Surface Sci. 2 (1964) 527. 9) A. B. Wintorbottot~, Optical Studies of Metal Surfaces, Kgl. Norske Videnskab. Selskabs Skrifter (I955) 1 (F. Bruns Bokhandel, Trondheira, 1955L 10) R. J. Archer, ref. 1, p. 255