Electrochemical and Solid-State Letters, 4 (1) C5-C7 (2001)
C1
1099-0062/2001/4(1)/C5/3/$7.00 © The Electrochemical Society, Inc.
Selective Deposition of Thin Copper Films onto Silicon
with Improved Adhesion
L. Magagnin,* R. Maboudian, and C. Carraroz
Department of Chemical Engineering, University of California, Berkeley, California 94720-1462, USA
A novel copper deposition method has been developed to plate silicon surfaces. Continuous copper films are obtained galvanically
on p- or n-type, single- or polycrystalline silicon. The films possess homogeneous structure, smooth surface, and improved adhesion
to the substrate. The plating bath comprises an aqueous solution containing a copper compound, ascorbic acid, ammonium fluoride,
and an antistress agent. With this process, the use of seed layers to improve adhesion between metal and semiconductor is avoided.
© 2000 The Electrochemical Society. S1099-0062(00)07-069-3. All rights reserved.
Manuscript submitted July 17, 2000; revised manuscript received October 2, 2000. Available electronically November 16, 2000.
Metallization plays a key role in the production process of integrated devices. Recently, copper (Cu) has been proposed as an alternative material to aluminum to address the need for metallic thin
films with low resistivity and high electromigration resistance.1 As a
consequence, great attention has been paid to electrochemical
processes, which enable the deposition of metallic copper at low
temperatures with low costs. Copper electroless deposition has been
shown to selectively plate silicon (Si) substrates and structures with
high aspect ratios and large structural heights.2,3 However, this
method requires a pretreatment to activate the surface. Selectivity is
achieved only to the extent that the activating treatment can be made
selectively. In addition, many studies have also been carried out on
the galvanic deposition of copper from fluoride containing solutions.4-11 In contrast to electroless deposition, galvanic displacement
deposition requires no prior activation of the surface and is truly
selective to silicon surfaces. Thus, it provides an attractive deposition method for copper interconnects or seed layers for subsequent
metallization.4 Galvanic displacement is also a promising avenue for
the integration of metals in micromechanical devices, due to its conformal nature and high substrate selectivity.
Despite these attractive features, a crucial issue in the aforementioned processes is the lack of adhesion of copper to the Si substrate,
which may severely constrain their application.12 In particular, copper films deposited by galvanic displacement fail the qualitative
Scotch tape test. In this paper, we report a process for the galvanic
deposition of copper onto silicon from fluoride-containing solutions.
Thin copper films with reflective and smooth surfaces and excellent
adhesion to silicon are obtained. Results on plating of microelectromechanical systems (MEMS) after release are also presented.
Polycrystalline silicon and single crystalline Si(100) and Si(111),
p- or n-type, and analytical grade chemicals were used. Samples
were ultrasonicated in acetone and, after drying with nitrogen flux,
etched in concentrated hydrofluoric acid for 10 min. The hydrogenterminated surfaces thus obtained were rinsed, dried, and immersed
in the plating solution. The following additives were used to prepare
the aqueous solution: ammonium fluoride (NH4F 40%) 50 vol %,
copper sulfate (CuSO4·5H2O) 0.01 M, ascorbic acid (C6H8O6) 0.01
M, sodium potassium tartrate (KNaC4H4O6·4H2O) 0.005 M, and
methanol 30 vol % (percentages are referred to the final solution volume).13 The pH of the solution was 7.5. Room temperature (25°C)
and gentle agitation of samples were used. Samples were finally
rinsed in deionized water and dried with nitrogen flux.
The adhesion of the copper film to the substrate is strongly related to the presence of ascorbic acid in solution. The absence of the
acid results in the formation of a copper film which fails the standard
scotch tape test. Because we have observed a similar effect when
ascorbic acid was substituted with fumaric acid, and both acids are
* Electrochemical Society Student Member.
z E-mail: [email protected]
known hydrogen scavengers, we believe that good adhesion is promoted by preventing hydrogen evolution at the silicon-copper interface.14 Sodium potassium tartrate and methanol are added to the
plating solution to lower the internal stresses of the film. They are
not essential for adhesion at short plating times.
Copper nucleation and growth on silicon have been studied with
scanning electron microscopy (SEM), atomic force microscopy
(AFM), and X-ray diffraction (XRD). Figure 1 shows two SEM
images of the morphology of the copper film on p-type Si(100) at
different plating times. From these images, one observes that, after
Figure 1. SEMs of the copper film on p-type Si(100) after (a) 10 s and (b) 1
h of plating.
C2
Electrochemical and Solid-State Letters, 4 (1) C5-C7 (2001)
Figure 3. SEM of the cross section of the copper film on p-type Si(100) after
2 h of plating. Film thickness is estimated to be ~200 nm.
Figure 2. Contact mode AFMs (1 x 1 µm, applied load in the range of 10-15
nN) of the copper film on p-type Si(100) after (a) 10 s, (b) 1 min, and (c) 2 h
of plating. RMS roughness (a) 2, (b) 2.7, and (c) 8.1 nm.
10 s of deposition, the substrate is completely covered with a copper
film, as a consequence of the coalescence of a large number of small
clusters. The surface of a thicker film, after 1 h of plating, shows a
homogeneous granular morphology with formation of small mounts.
The nucleation and growth mechanism is confirmed by the AFM
images in Fig. 2. A 3D island growth mechanism with nucleation of
metal clusters and subsequent growth of a film is observed. An
extremely fast nucleation with a large number of nuclei, as shown in
the AFM image after 10 s of plating, characterizes the initial displacement of the metal. Nuclei increase in size with plating time and
coalesce in a metallic homogeneous film, as in the image obtained
after 1 min. As one observes in the image after 1 h of plating, the
deposition proceeds with a 3D growth. This 3D structure is evidently permeable to fluoride ions, which explains why such long plating
times (and thick deposits) can be obtained by the displacement reaction. The root mean square roughness, measured with AFM, is in the
range 10-20 nm after 2 h of plating.
The SEM micrograph in Fig. 3 shows the cross section after fracture of a thick copper film, after 2 h of plating. The film is compact
and homogeneous with columnar-shaped formations. The thickness
is about 200 nm. The surface morphology reveals the 3D growth,
although the film appears optically smooth and reflective. From
SEM-based thickness measurements, the rate of growth is estimated
to be about 100 nm/h and adhesion of the film is maintained even
after 4 h of deposition. The XRD pattern of a copper thick film, after
2 h of plating, on p-type Si(100) shows a strong preferential orientation for the (111) plane, the closed-packed plane for the face-centered cubic structure, and a peak for the (100) plane, related to the
influence of the substrate. Qualitative adhesion tests, i.e., Scotch
tape test and scratch test, have been carried out on copper films
grown on different substrates and for varying film thickness.
Commercial 3M scotch tapes and industrial tapes were used and
scratching tips with different hardness tested, including stainless
steel and tungsten. Peeling of the deposited copper film was never
observed. Independent of the substrates (i.e., p- or n-type, single- or
polycrystalline silicon), the copper films exhibit a very high adhesion to the substrate and resistance to scratching, particularly compared to films obtained according to the published literature.4-11
The particular structure of the copper film, likely due to the 3D
growth, has a strong influence on the electrical resistivity of the
films as deposited. Compared to a sputtered copper film on a titanium nitride seed layer, which exhibits resistivity of about 300 mΩ/h,
the displaced copper as deposited is characterized by resistivity values over ten times higher. However, after heat-treatment at 150°C for
20 min under nitrogen flux, the displaced copper film exhibits a
Electrochemical and Solid-State Letters, 4 (1) C5-C7 (2001)
C3
coverage is always observed, indicating that the coating process is
conformal, as expected. The scanning electron micrograph in Fig.
4a shows a detail of a coated MEMS structure including an interdigitated comb drive and polysilicon interconnects on a silicon nitride
substrate. The selectivity of the coating process to silicon is confirmed by the SEM/energy-dispersive spectrscopy (EDS) micrograph (Fig. 4b), which shows copper signal coming from polysilicon
regions only.
In conclusion, a process for the deposition of copper onto silicon
by galvanic displacement has been described. The copper films are
smooth, homogeneous, and reflective. Improved adhesion of the
metal to the semiconductor and low electrical resistivity after heattreatment have been obtained. The galvanic displacement of copper
according to the described method appears promising for the deposition of copper both in interconnect technology and in micromechanical devices after release.
Acknowledgment
The authors thank Professor P. L. Cavallotti for SEM and XRD
measurements.
Dr. Carraro assisted in meeting the publication costs of this article.
References
Figure 4. (a) SEM showing a detail of a polysilicon MEMS structure on a
silicon nitride substrate. The structure was coated with copper by galvanic
displacement. (b) SEM/EDS micrograph of the same area revealing that only
polysilicon features are coated.
resistivity comparable to that of sputtered copper and adhesion is
maintained. An added benefit of the heat-treatment is that, according
to published patent, a copper silicide layer is formed at the
metal/semiconductor interface; the silicide layer behaves as a perfect
seed layer and diffusion barrier.15,16
The coating process has been used to plate polycrystalline silicon
micromachined devices after the release step. Excellent sidewall
1. J. S. H. Cho, H. Kang, S. S. Wong, and Y. Shacham-Diamand, MRS Bull., 18,
31 (1993).
2. S. Dhingra, R. Sharma, and P. J. George, Solid-State Electron., 43, 2231 (1999).
3. S. Lopatin, N. MacDonald, J. Chen, and S. Adams, Abstract 948, The
Electrochemical Society and the Electrochemical Society of Japan Meeting
Abstracts, Vol. 99-2, Honolulu, Hawaii, Oct 17-22, 1999.
4. M. K. Lee, H. D. Wang, and J. J. Wang, Solid-State Electron., 41, 695 (1997).
5. O. M. R. Chyan, J. J. Chen, H. Y. Chien, J. Sees, and L. Hall, J. Electrochem. Soc.,
143, 92 (1996).
6. J. S. Jeon, S. Raghavan, H. G. Parks, J. K. Lowell, and I. Ali, J. Electrochem. Soc.,
143, 2870 (1996).
7. S. G. dos Santos F, L. F. O. Martins, P. C. T. D’Ajello, A. A. Pasa, and C. M.
Hasenack, Microelectron. Eng., 33, 59 (1997).
8. S. G. dos Santos F, A. A. Pasa, and C. M. Hasenack, Microelectron. Eng., 33, 149
(1997).
9. M. K. Lee, J. J. Wang, and H. D. Wang, J. Electrochem. Soc., 144, 1777 (1997).
10. G. Li, E. A. Kneer, B. Vermiere, H. G. Parks, S. Raghavan, and J. S. Jeon,
J. Electrochem. Soc., 145, 241 (1998).
11. X. Cheng, G. Li, E. A. Kneer, B. Vermiere, H. G. Parks, S. Raghavan, and J. S.
Jeon, J. Electrochem. Soc., 145, 352 (1998).
12. G. Oskam, J. G. Long, A. Natarajan, and P. C. Searson, J. Phys. D: Appl. Phys.,
31, 1927 (1998).
13. L. Magagnin, C. Carraro, and R. Maboudian, Invention and Technology
Disclosure Form, Office of Technology Licensing, University of California,
Berkeley (2000).
14. See, for example, J. Xu, and R. B. Jordan, Inorg. Chem., 29, 2933 (1990).
15. A. M. Osama, U.S. Pat. Appl. N. EP0419763 IBM (1991).
16. K. Lia, S. Yuan-Chen, and A. M. Osama, U.S. Pat. Appl. N. EP0472804(1992).