Nanostructures formed by displacement of porous silicon with copper: from nanoparticles to porous membranes
© Bandarenka et al.; licensee Springer. 2012
Received: 4 May 2012
Accepted: 4 August 2012
Published: 23 August 2012
The application of porous silicon as a template for the fabrication of nanosized copper objects is reported. Three different types of nanostructures were formed by displacement deposition of copper on porous silicon from hydrofluoric acid-based solutions of copper sulphate: (1) copper nanoparticles, (2) quasi-continuous copper films, and (3) free porous copper membranes. Managing the parameters of porous silicon (pore sizes, porosity), deposition time, and wettability of the copper sulphate solution has allowed to achieve such variety of the copper structures. Elemental and structural analyses of the obtained structures are presented. Young modulus measurements of the porous copper membrane have been carried out and its modest activity in surface enhanced Raman spectroscopy is declared.
KeywordsPorous silicon Copper Displacement deposition Nanostructures
Despite its long-standing discovery, porous silicon (PS) has been attracting a great attention as a breakthrough material with exceptional characteristics for microelectronics, integrated optoelectronics, microelectromechanical systems (MEMS), layer transfer technology, solar and fuel cells, biomedicine, etc.. Partially, this is because of the opportunity to easily vary the properties of PS in wide ranges by introducing different materials into its pores. Indeed, the use of an array of ordered pores as a template can provide the creation of specific composite structures with novel electrical, optical, magnetic, plasmonic, and other features[3–5]. Among many, an interest on nanocomposites fabricated by immersion of PS into aqueous solution of copper salt has not been attenuating for more than a decade[6–8], though the mechanism of Cu immersion deposition on bulk monocrystalline silicon had been studied much earlier because wet chemical cleaning in H2O- and hydrofluoric acid (HF)-based solutions containing an extremely low concentration of copper ions has resulted in the adhesion of copper contaminants on Si wafers. To prevent the presence of undesirable Cu traces on Si, the mechanism of copper adhesion has been studied and understood[10, 11]. Because of their positive redox potential, copper ions have been found to attract electrons from silicon, resulting in simultaneous copper reduction and Si oxidation. In that way, the nucleation and growth of Cu precipitates with diameters of few nanometers occur[11–13]. Later, a number of studies have been carried out to fabricate copper films by immersion of bulk silicon in a solution with higher concentration of copper ions[14, 15]. It promotes growth of copper precipitates to islands which then increase in sizes and coalesce together forming a quasi-continuous film[15, 16]. Such films have been representing as suitable candidates for IC interconnections and MEMS technology due to their low resistivity, their selectivity of deposition between silicon and dielectric mask, as well as the simplicity and cost-effectiveness of the fabrication process, which does not require high temperature, special complex equipment, and illumination[14, 15]. However, to obtain reliable adhesion of immersion Cu films to Si, it is necessary to use 350°C in annealing. To solve the problem in an easier way, the authors of the papers[17, 18] have proposed to form a thin layer of PS before copper deposition. Deep penetration of copper atoms into porous layer during immersion deposition results in the formation of a Cu/PS composite, providing several times of increasing copper film adhesion. This is another advantage of the immersion method because during evaporation or sputtering, depositing copper atoms are located at the entrances of pores.
On the other hand, PS is traditionally used as a direct-bandgap semiconductor (in contrast to bulk indirect-bandgap Si) that allows integration of optoelectronic devices with Si technology. Actually, PS is known to demonstrate visible, red photoluminescence, but introduction of copper nanoparticles (NPs) in its porous volume promotes obtaining emission in other wavelengths. Prospects of easy variation of the PS luminescence have significantly increased the interest in studying copper immersion deposition in PS. The subsequent research established the influence of the inner composition of PS (SiO2, SiHx, OySiHx) on chemical reactions and the inhibition of deposition in the presence of halogen ions. Other specific features of immersion Cu/PS is an activity in surface enhanced Raman spectroscopy (SERS) which is one of the most sensitive methods in analytical chemistry, biomedicine, ecology, etc. Unfortunately, Cu/PS SERS-active substrates have not been widely studied yet in comparison with other competing porous substrates based on anodic aluminum oxide.
All mentioned works on the immersion deposition of Cu on PS have applied simple aqueous solutions of copper salts accompanied by the formation of SiO2 under a copper deposit which stops the redox reaction and prevents the dissolution of porous template. At the same time, it has limited the number of nanoscale structures which could be formed by the immersion method. Recently, turning back to the study of copper contaminations on bulk Si, addition of HF to the solution for copper deposition on PS has been proposed. HF allows SiO2 removal and continuous deposition of copper, as well as silicon dissolution. This process is usually called displacement deposition because of the nonstop substitution of the substrate's atoms with the metal's atoms. Some previous works have shown that copper deposits by displacement on the surface of PS in the form of crystalline NPs. The pore channels limit the size of NPs, while on the outer PS surface, copper particles can be an order of magnitude greater. In fact, the final material of copper displacement deposition on PS represents the layer of the Cu/PS nanocomposite covered with quasi-continuous copper film. The initial stages of deposition are accompanied by the formation of copper particles of 2-nm diameter inheriting the crystallographic orientation of the PS skeleton. Further copper particles growth leads to the (111) prevalence orientation of the copper deposit[24, 26]. In, the outer surface of PS of 55% porosity has been used as a template for the growth of copper particles of controllable sizes. However, the authors have been faced with the fact that copper deposits according to the island growth mechanism of thin films (like in the case of bulk Si). As the level of PS surface coverage with copper particles reached the critical value, gradual formation of quasi-continuous Cu film has been observed. To achieve the growth of separated Cu particle arrays on the PS substrate of required dimensions, the authors reduced the solution temperature and used alcohol as the wetting agent. Nevertheless, easy and controllable managing of the morphology and structure of the Cu deposit on PS is still an urgent target as it helps to develop new, effective, and simple technology both for ohmic contact and Schottky-like structures, and conductive films of extremely high adhesion to Si and PS[17, 26]. It is notable that to date, positive results of the displacement method might be presented. Combined technology of double-layered PS and copper displacement deposition has been successfully tested for the measurement of PS mechanical strength and manufactured to form compliant contact arrays for probe cards. Remarkably, a spiral of thick, porous copper membrane on a flexible silicone substrate reported to be fabricated by displacement technique from PS has been found to promote drug electroporation. However, fabrication regimes and the morphology of the original PS have not been opened as the authors have referred to the paper which reported the full conversion of PS of only 1-μm thickness into a copper layer, whereas further successful application of such metal porous structures strongly depends on the detailed understanding of its formation mechanism and properties. One of the strong needs is mechanical strength data of the membrane for electroporation because to be in good contact with the surface of treated living tissue, it should have flexible stability.
In the present work, we have proposed to vary parameters of PS to fabricate by displacement technique copper NPs of controllable dimensions as well as thick, porous copper membrane. We have carried out measurements of the Young modulus of the obtained copper membrane. In addition, modest SERS activity of the copper porous membrane has been declared.
Si wafer characteristics and PS parameters
Si wafer doping
Si wafer resistivity, ρ
Porosity of PS, p
Thickness of PS, d
50 to 55
80 to 85
80 to 85
60 to 65
2.5 to 3
After PS formation, the HF solution was removed, and the electrolytic cell was thoroughly rinsed with deionized water for 3 min and then with C3H7OH for 5 min. The cell was filled with the solution for copper deposition for 4 to 7200 s at 25°C. We used two solutions for the copper displacement deposition: (1) basic CuSO4·5H2O + 0.005 M HF (45%) aqueous solution and (2) 0.025 M CuSO4·5H2O + 0.005 M HF (45%) + 0.1 M C3H7OH aqueous solution of improved wettability. To stop the deposition process, the solution was poured from the cell. Finally, the sample with a Cu/PS layer was rinsed three times for 30 s with deionized water, dried in air for 30 min, and removed from the cell.
The morphology and structure of the samples were studied with a scanning electron microscope (SEM; Hitachi S-4800, Chiyoda-ku, Japan) with a resolution of 1 nm. The elemental composition of the samples was determined using a Cambridge Instruments Stereoscan-360 SEM (Cambridge, UK) with a Link Analytical AN 10000 energy-dispersive X-ray analyzer (Redwood, CA, USA). The diameter of the focused electron beam was no more than 1 μm, the atomic mass accuracy did not extend 0.1%, and the depth of the analysis was 1.3 to 1.5 μm under 20 keV. The equipment used to conduct electrochemical processes was the AUTOLAB PGSTAT302n potentiostat/galvanostat (Utrecht, The Netherlands). Gravimetric method was applied to determine the porosity of PS and copper membrane. Mass measurements were erformed with a Sartorius CP225D micro/analytical electronic balance (Goettingen, Germany). The instrumental mass error was about 10 μg. The phase composition of the samples was determined by X-ray diffraction (XRD) using CuKα radiation (X-ray wavelength λ = 0.15406 nm).
SERS activity of the porous copper membrane was tested using water-soluble cationic Cu(II)-tetrakis(4-N-methylpyridyl)porphyrin (CuTMpyP4) as an analyte compound. For the SERS measurements, a 0.02-ml drop of the 10−6 М porphyrin solution was poured on the porous copper membrane. After drying in air, a round spot of 1-cm diameter was observed on the copper surface. Raman spectra were registered with the spectrometers SpectraPro 500 I and T64000 (Jobin-Yvon, Milan, Italy), equipped with CCD detectors. The sources of continuous excitation were a Liconix helium-cadmium laser (λ = 441.6 nm; Santa Clara, CA, USA) and a semiconductor laser (λ = 532 nm). The accuracy of the frequencies in the spectra did not exceed 1 cm−1. SERS spectra were recorded upon continuous rotation of the sample for signal averaging and prevention of porphyrin destruction.
Results and discussions
The deposition process was visually accompanied by gradual color change of the surface of PS from black to red which is typical for copper. Gas bubbles released from the surface of the sample were also observed. The activity of the gas evolution was weakened with the increase of deposition time. According to, the released gas is hydrogen which is a product of the redox reactions. The decrease of its evolution means a slowing of the process.
Formation of copper particles of the nanoscale range on the outer PS surface requires the use of PS of only 1-μm thickness. A thin porous layer allows minimizing the amount of reagents and deposition time needed for the growth of NPs. However, that limited the thickness of the converted porous copper film just to 1 μm. In trying to study the properties of such porous copper, we separated it from the Si substrate, but the metallic film had too weak mechanical strength and, in free form, represented pieces of about 25-mm2 area. Thus, to further work with the free porous copper, the increase of its thickness was highly required.
To reveal the elemental composition of the membrane, EDX analysis of the cross section, top side, and bottom side were carried out (Figure8d,e,f). EDX scan of the cross section was attempted as well, but it was impossible to correctly focus the 1-μm electron beam on the non-flat surface of the agglomerates. To overcome doubts on the elemental composition, EDX analysis was performed in ten different points of the cross section, and each showed 97 to 99 at.% of Cu content. An example of point EDX in the cross section is presented in Figure8e. Figure6b,c confirms the copper nature of the obtained membrane. Overall, the membrane uniformly contains 95 to 99 at.% of copper with small amounts of oxygen and carbon. The maximum content of Si atoms was 0.1%, i.e., it might be declared that the obtained membrane represents the copper material. The gravimetrically determined porosity of the membrane was 60% to 65% in comparison with bulk copper.
Based on the results of SEM and EDX analyses, we propose the following phenomenological model of the formation of porous copper membrane. On the stage of full impregnation of PS with the solution, Cu NPs nucleate and grow on the surface of PS skeleton. As metal deposition was carried out simultaneously with dissolution of Si pillars, PS skeleton was converted into bottom spongy copper layer. The supposition might be proved by equality of the thickness of the original PS4 to that of the bottom copper layer (2.5 to 3 μm). In our opinion, new copper NPs grow and coalesce on the outer surface of the spongy copper layer. In that way, a layer of huge copper agglomerates is formed, whereas stresses on the Si/Cu membrane interface exceed over the interaction force between silicon and copper atoms when the copper membrane separates from the substrate as observed during the experiment. Detailed understanding of the porous copper membrane formation requires more careful in-depth study which is under the scope of the future paper.
Cu nanoparticles, quasi-continuous copper films, and free porous copper membranes were fabricated by displacement deposition of copper on PS templates from the aqueous solution of copper sulphate with HF and C3H7OH additions. It was found that the PS porosity and morphology as well as the time of deposition define the structural type of the Cu deposit.
The layers of mesoporous silicon of 1-μm thickness and 80% to 85% porosity represent a template for the fabrication of separated copper NPs of 20- to 280-nm diameter. Managing the Cu NP sizes is provided by time variation of PS immersion in the copper salt solution.
Copper displacement deposition on mesoporous silicon of 1- to 7-μm thickness and 50% to 85% porosity for more than 180 s allows formation of quasi-continuous copper films up to 500-nm thickness.
Macroporous silicon of 3-μm thickness and 60% to 65% porosity immersed into copper salt solution of improved wettability (with isopropanol additive) for 7200 s completely converts into porous copper membrane.
Young modulus of porous copper membrane depends on the porosity and has anisotropic nature in perpendicular and parallel directions. The measurements are useful for the further development of flexible and elastic materials for electroporation in biomedicine.
The demonstration of the modest SERS activity of the porous copper opens new prospects of Cu-based substrates for traces of substance detection. That might decrease the costs of SERS analysis in comparison with traditionally used substrates based on gold and silver.
HB is a research scientist and is going to defend her Ph.D. thesis this fall 2012. SR is a junior researcher and a second year Ph.D. student. Both of them work in the R&D laboratory ‘Materials and Structures of Nanoelectronics’ (Micro- and Nanoelectronics Department, BSUIR). The head of the mentioned laboratory is VB, Ph.D., who is an assistant professor and teaches the courses ‘Nanomaterials’ and ‘Microelectronic Technology’. Sc. Dr. Professor AS is the head of the R&D laboratory ‘Information Display and Processing Units’ (Micro- and Nanoelectronics Department, BSUIR). AP, Ph.D., is a research scientist. ST, Ph.D., is a leading researcher in the R&D laboratory ‘Photonics of Molecules’ (B.I. Stepanov Institute, NASB). PN is a research scientist and is in the last year of being a Ph.D. student. Assistant Professor MB teaches the course ‘Microelectromechanical Systems.’ The two last authors are fellows of the Department of Information Engineering, Electronics and Telecommunications at the University “Sapienza.”
Energy-dispersive X-ray spectroscopy
Scanning electron microscope
Surface enhanced Raman scattering
This research was partially supported by the Belarusian Foundation for Basic Research under Project T11OB-057, by Rise Technology S.r.l. (Roma, Italy), and by the European Union under the project ‘BELERA.’
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