The strength of the bond between metals and SiO2 was studied by measuring the static coefficient of friction of metals contacting alpha-quartz in ultrahigh vacuum. It was found that copper with either chemisorbed oxygen, nitrogen, or sulphur exhibited higher contact strength on stoichiometric SiO2 than did clean copper. Since the surface density of states induced by these species on copper is similar, it appears that the strength of the interfacial bond can be related to the density of states on the metal surface

Pepper, S. V.

English

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https://ntrs.nasa.gov/search.jsp?R=19780011596 2020-03-22T04:19:17+00:00Z
NASA TECHNICAL
MEMORANDUM
NASA TM-78838
1
00
M
00
00
ti	
(NASA-T"".-7RPIR)	 S1tFAR STPENGTH OF MFT y L -	 N7P-11539
,iP? CO"TACTS ("'AG1)	 C p HC A02/MF A01
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CSr-'L 20K	
T1ricIaS
d	 ' 3/39 01145 4Z
SHEAR STRENGTH OF METAL - Si02
 CONTACTS
by Stephen V Pepper
Le1Ats Research Center
Cleveland Ohio 44135
TECHNICAL PAPER to be presented at the
International Topical Conference on the Physics of
SiO 2 and Its L;terfaces
YT,rktown Heights, New Jersey, March 22-24, 1978
J`v	 ^.^
1	 ,^
R	
-,8-4
SHEAR STRENGTH OF METAL - SiO 2 CONTACTS
Stephen V. Pepper, NASA - Lewis Research Center
21000 Brookpark Rd., Cleveland, Ohio 44135
M
	 ABSTRACT
U)
w	 The strength of the bond between metals and S102 is studied by measuring the
static coefficient of friction of metals contacting a-quartz in ultrahigh
vacuum. It was found that copper with either chemisorbed oxygen, nitrogen
or sulphur exhibited higher contact strength on stoichiometric S10 2 than
did clean copper. Since the surface density of states induced by these
species on copper is similar, it appears that the strength of the interfacial
bond can be related to the density of states on the metal surface.
TNTPnn1TCTTnN
The useful electrical properties of the interface between different solids
has led to the fabrication of solid state electronic devices and a parallel
investigation of the phvsical basis of these properties. The investigation
into the physical nature of the adhesive forces between solids required in
fabrication of electronic devices has not progressed as rapidly. This lack
of progress is partly due to the experimental difficulty of obtaining a
measure of interfacial strength (1, 2) that can be interpreted as due to only
one of the man y different forces that can act between solids (3). Adhesion
problems that have arisen in device fabrication have been dealt with rather
empirically, without fundamental understanding in which to act.
A system that should benefit from a fundamental investigation of interfacial
strength is the metal-Si02 system. This s y stem is considered to be a
of
	
in that noble metals do not adhere well and must be joined to 5102
with intermediate layers of Ti or Cr.
In this paper changes in the shear strength of Cu-S10 2 contacts induced by
chemisorbed 0, S and N on Cu are studied. The shear strength has been shown
to be correlated to, thin film adhesion via the popular scratch test (2, 4).
This correlation is the principle _justification for associating the results
obtained here with what is generally considered to be adhesion. The use of
0, S and N is motivated by recent work (5) indicating that they induce similar
changes in the Cu surface valence electron density of states. 	 Since it is
the valence electrons of a solid which determine the chemical interaction of
the solid with other species, the contact strength should change in similar
ways due to the presence of those elements on the Cu of in fact the adhesive
bonding between Cuand SiO 2 is due to a chemical interaction.
ORIGINAL I'AV E•: 1.
OF PUOR (QUALITY (I
2
APPARATUS
Contact shear strength is measured in ultrahigh vacuum by pressing a copper
sphere onto an SiO2 flat and then subjecting the contact to a tangential
force. The maximum tangential force that the contact can support divided by
the normal force yields a static coefficient of friction, ps. Changes in ps
due to chemisorbed 0, S and N on Cu, from the value obta_ned for the clean
surfaces, are the principle experimental results. This method has been used
to study metal - Al 2 03 co ntact strength (6). _Fip,. 1 depict s the shear ap-
paratus.	 _ _
v ION GUN
^FLE)I PIVOT	 'METAL SPHERETRANSLATION 	 QUARTZ FIAT
ROTATION FILAMENT--,
W	 ION SOURCE `•:%'`^	
— PIEZOELECTRIC
1REF. 71	 FORCE TRANSDUCER
GRID — 	BALL SLIDE
Figure 1.	 Shear apparatus in ultrahigh vacuum.
The load of the metal on the S10 2
 flat is provided by vertical force W. A
differential screw provides tangential motion and tangential force is measured
by the piezoelectric force transducer in vacuum. The sphere is retracted
(dotted line) to allow specimen cleaning by Ar +
 bombardment of the 510 2 (ion
gun) or of the sphere by the electron bombardment ion source (7). This source
also "activates" N2 which does not chemisorb on Cu in molecular form (5). For
"activated" chemisorption the metal sphere is grounded and the source operated
at 5 x 10- 5 torr N2 with the ion pump on.
Surface analysis with a single pass cylindrical mirror analyzer with coaxial
electron gun was performed in another part of the vacuum chamber, but with the
specimens and ion sources removed from the shear apparatus. Auger electron
spect roscopy (AES) and energy loss spectroscopy (E,LS) for S10 2 were used to
chara--terize the surfaces aft ,2r ion bombardment and after exposure to gases.
Since the same surface preparation procedures were used both prior to contact
and prior to surface analysis, it is assumed that the analysis is valid for
the contacting surfaces. Polycrystalline metal and single crystal a-quartz
were used.
SURFACE CHARACTERIZATION
After Ar + bombardment Lt he Cu _Auge r _, es trum in_Fig._ 2__resulteed.—
So O. 31
Figure Z Auger spectrum of clan Cu and adsorbate Auger peaks. I
C
2v
0/
I
W
W
ZO
_...1
	 _ _ 1 -_	 . ^. _
The usual contaminants are absent and there is ome Ar. Subjecting this clean
Cu surface to exposure of either 10 3 L 02 or 10 L H2S yields either the 0 or
S Auger peaks indicated. Subjecting the clean Cu surface to activated N2 for
3 min. yields the N Auger peak indicated. Analysis of the fine structure of
the adsorbate peaks showed good agreement with those obtained by Tibbetts et al
(5). Assuming the peak-peak heights are proportional to elemental concentration,
taking into account the linear energy dependence of the sensitivity of the CMA
and the Auger yields of the elements (8), it is found that the elemental con-
centrations for 0, N and S are in the ratio 1:.73:.77. Thus the relative
concentrations are quite similar. The absolute concentrations are probably
on the order of ^ monolayer (5).
The Auger spectrum of Ar + bombarded 5102 indicated that the surface was free
of impurities and the Si L'V-V structure is shown in Fig. 3a.
14 92 eV
	
64 eV 4	 64 eV^
78 eV
78 eV
	
ta)	 p)
15	 10	 5	 0
ENERGY LOSS. eV
Figure Q Energy loss spectrum of S '0 2 la' after Ar+bombardment and tb) after Oz bombardment.
Figure 3. Si LVV Auger peal Ia) after
Ar' bombardment and bbl after Op
bombardment.
The virtual absence of the 92eV transition indicates that the surface was not
reduced to elemental GI by the Ar + , However, the structure at 78eV is absent
after 1 keV 0-) +
 bombardment, Fig. 3b. Since the latter surface is probably
stoichiometric SiO 2
 (9, Fig. 3), it appears that Ar + reduces the surface, but
not ,till the way to elemental Si.
	 'Phis is corroborated by ELS, Fig. 4. A
comparison with the second derivative ELS of 5102 (9, Fig. 5) indicates that
the spectrum of Fig. 4b is close to that of stoichiometric S102, but that
Fig. 4a contains large peaks at 4.9eV and 7.1eV that have been attributed to
the presence of a "surface phase of approximately Monoxide composition" (10).
The stoichiometric surface is the better defined of the two and ma y be easier
to handle analytically in a future theory of the SiO-) - metal interface. Both
surfaces were investigated because the reduced surface is generated b y the
sputter-etch step in thin film deposition and thus the results may be applic-
able to thin film adhesion.
ORIGINAL PAGE IS
tlr PU()It QUAUry
old
i
i•
II
4
RESULTS AND DISCUSSION
The results for stoichiometric St0 2 are presented first. Both surfaces were
cleaned by Ar+ bombardment, the S10 2 was oxidized by 0 2+ bombardment and
finally the Cu was recleaned to prepare a clean Cu - S102 system. Although
the static friction coefficient varied from one experimental run to another
(due perhaps to differing contact geometries on the metal sphere), the
changes in static friction following the exposures were quite reproducible.
In Fig. 5 are depicted force-displacement curves for the clean contact and
--- ---- -----
LOAD
 • 50-gmt-
SLIDING
E	 BEGINS,---____ _- 	
►+SNm
4r
cu 4
S	 ,
JQ 	 IG
1
[rDISPLACEMENT—
CLEAN	 AFTER AFTER AFTER
0 2	N2	H2S
Figure 5. Force-displacement curves for contact on
stoichiometric SiO2.
-Ii
for contact after exposure to 02, N 2 and H 2 S. The clean contact has u s - .3
and significant increases over this value are exhibited after all the ex-
posures. The largest increase was due to N 2 . Smaller increases were found
after 02 and H2S, even though the force-displacement curves that illustrate
the approa=h to sliding are (reproducibly) different. The contact strength
on the reduced Si0 2 was somewhat different: the clean contact strength and
the effect of N 2 was the same as for the stoichiometric 5102, but little, if
any, increase waG observed after the 0•) or H 2 S exposures. Motivated by recent
work which showed that the density of states induced by 0, N and S on Ag were
also quite similar to those observed on Cu (11), experiments were performed
with a Ag sphere. For this relativelv inert metal, all adsorption on the
clean Surface was by electron beam activation. The results were similar to Cu,
although here increased contact strength was observed only on the stoichio-
metric S10 ? and not on the reduced surface, even for N2 exposures.
In considering these results, first note that the increased strength due to
the presence of these adsorbates is contrary to the view that maximum inter-
facial strength is obtained by removing all foreign species from both surfaces
prior to joining. Although this may be the case in joining similar materials
as in metal-metal contacts, it is certainly not the case in the situation
considered here. Secondly, a previous study of metal - Al 2 03 contact strength
(b) has assumed the need for two separate theories of interfacial strength to
understand the effect of adsorbed oxygen on the metal - one for clean metal -
Al203 and one for metal oxide - Al203. Here, however, we deal not with the
copper oxide but with a partial monolayer chemisorbed surface species. Thus
one theory of contact strength should be sufficient - for copper and its sur-
face modifications - rather than for the very different entities Cu and Cu20
in contact with S102.
r
i-
AJ
The adsorbates that change the valence electron density of states in similar
ways have led to similar changes in contact strength only on stoichiometric
S10q. A different situation prevailed on reduced 510 21 where only N2 in-
creased the contact strength. The contact strength cannot be understood
solely in terms -)f the surface properties of one of the partners and requires
a more complicated, as yet undeveloped, understanding of the interaction. An
important finding, however, is the effect of adsorbed N. This large effect
was not anticipated on grounds other than the induced valence electron density
Of states and thus lends some confidence to this approach to understanding
contact strength.
Finally, the enhanced bonding observed here can have the practical effect of
improving thin film adhesion. This has been noted for some time for rather
thick intermediate oxide films (2) but results here show that the enhancement
may also be possible by monolayer concentrations. This has in fact been
observed by the author in preliminary experiments with the Cu-N-S102 system
by R. 17 . sputter deposition in a partial pressure of N2.
REFERENCES
1. Tabor, D.:	 (1975), in Surface Physics of Materials, vol. II,
(J. M. Blakely, ed.), Academic Press, New York.
2. Campbell, D. S.: (1970), in Handbook of Thin Film Technology,
(L. I. Maissel and R. Glang, eds.), McGraw - Hill, New York.
3. Krupp, H.: Particle Adhesion Theory and Experiment, Advan. Colloid Inter-
face Sci., 1, II1, (1967).
4. Benjamin, P. and Weaver, C.: Measurement of Adhesion of Thin Films,
Proc. Roy. Soc., London, A254, 163, (1960).
5. Tibbetts, G. G., et al: Electronic Properties of Adsorbed Layers of
Nitrogen, Oxygen and Sulphur on Copper (100), Phys. Rev. B, 15, 3652,
(1977).
6. Pepper, S. V.: Shear Strength of Metal-Sapphire Contacts, J. Appl. Phys.,
47, 801, (1976).
7. Cuthrell, R. E. and Tipping D. W.: Sandia Laboratory Report SC-RR-72-
0783, (1972).
8. Meyer, F. and Vrakking, J. J.: Quantitive Aspects of Auger Electron
Spectroscopy, Surf. Sci., 33, 271, (1972).
9. Ibach, H. and Rower, J. E.: Electron Orbital Energies of Oxygen Adsorbed
on Silicon Surfaces and of Silicon Dioxide, Phys. Rev. B, 10, 710, (1974).
10. Rowe, J. E.: Photoemission and Electron Energy Loss Spectroscopy of
Ge02 and SiO.,, ^1. Phys. Lett., 25, 576, (1974).
11. Tibbetts, G. ` G. and Burkstrand, J. M.: Electronic Properties of Adsorbed
Layers of Nitrogen, Oxygen and Sulphur on Silver (111), Phys. kev. B,
16, 1536, (1977).
ORIGINAL PA(=1-' l"
OF POUR 12Lmxry
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Shear strength of metal - SiO2 contacts

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