Si nanopatterning by reactive ion etching through an on-chip self-assembled porous anodic alumina mask
© Gianneta et al.; licensee Springer. 2013
Received: 21 December 2012
Accepted: 4 February 2013
Published: 12 February 2013
We report on Si nanopatterning through an on-chip self-assembled porous anodic alumina (PAA) masking layer using reactive ion etching based on fluorine chemistry. Three different gases/gas mixtures were investigated: pure SF6, SF6/O2, and SF6/CHF3. For the first time, a systematic investigation of the etch rate and process anisotropy was performed. It was found that in all cases, the etch rate through the PAA mask was 2 to 3 times lower than that on non-masked areas. With SF6, the etching process is, as expected, isotropic. By the addition of O2, the etch rate does not significantly change, while anisotropy is slightly improved. The lowest etch rate and the best anisotropy were obtained with the SF6/CHF3 gas mixture. The pattern of the hexagonally arranged pores of the alumina film is, in this case, perfectly transferred to the Si surface. This is possible both on large areas and on restricted pre-defined areas on the Si wafer.
78.67.Rb, 81.07.-b, 61.46.-w
Si nanopatterning finds important applications in nanoelectronics, photonics, and sensors. Advanced techniques as electron beam lithography or focused ion beam milling can be used in this respect; however, they are both expensive and time consuming when large areas have to be patterned. The use of a masking layer either on the whole wafer or locally on pre-defined areas on the Si substrate can provide a good and cost-effective alternative to the above techniques. Porous anodic alumina (PAA) thin films on Si offer important possibilities in this respect. PAA films can be fabricated on the Si wafer by electrochemical oxidation of a thin Al film deposited on the Si surface by physical vapor deposition. The so-formed aluminum oxide (alumina) shows highly ordered vertical pores that can reach the Si substrate and can be used either as a masking layer for Si nanopatterning [1–4] or as a template for nanowire and nanocrystal growth within the pores. PAA films can be also used as the dielectric material in metal-oxide-metal (MIM) capacitors [5–7] and as the charging medium in non-volatile memory devices . PAA films can be formed either on large areas or on pre-selected small areas of the Si wafer. This is very useful in all the above applications. In Si nanopatterning, the Al film is first patterned and is then anodized to form the PAA mask. It is thus possible to pattern both small areas and very large areas on the Si wafer.
Perfectly hexagonal self-ordered PAA films were first reported on Al foils by Masuda and Fukuda in 1995 . Other works then followed, which focused on the variation of the main properties of such ordered PAA films, i.e., the cell size, pore diameter, and pore depth as a function of the anodization conditions (i.e., the acidic solution, the anodization voltage, and the anodization time used [10–12]).
For a perfect masking technology for Si nanopatterning, the development of optimized PAA films with tunable pore size and density on the Si wafer are needed. Perfect PAA layers are easily achieved on an Al foil [13, 14]. After their release from the Al substrate, free standing PAA membranes are fabricated. Such membranes were used in the literature for Si nanopatterning . However, the direct formation of the PAA mask on the Si substrate offers more flexibility in the etching process than free standing PAA membranes. The structural difference of PAA films on Si compared with similar films on an Al foil is mainly at the interface with the Si substrate. Anodization of the film on Si proceeds as in the case of the Al foil until the so-called barrier layer of the alumina film reaches the Si surface. At this stage, the barrier layer at each pore bottom is detached from the rigid Si substrate under mechanical stress, forming a thin capping layer over a void at each pore base [16, 17]. After the void and capping layer formation, if the electrochemical process is not stopped, it proceeds by oxidizing the Si surface, starting from the pore walls and continuing to fully oxidize the Si surface underneath the PAA film. In most of the applications, the anodization has to be stopped just after full Al consumption. The barrier layer at each pore bottom has to be removed so as to get pores that reach the Si surface.
In this paper, we applied optimized PAA thin films on Si with regular long range pore arrangement and we investigated the pattern transfer to the Si wafer using reactive ion etching (RIE) with three different fluorine gas mixtures: pure SF6, SF6/O2, and SF6/CHF3.
PAA films used in this work were fabricated by anodic oxidation of an Al film, deposited on Si by electron gun evaporation. The electrolyte used was an aqueous solution of oxalic acid, 5 w.t.%, and the process was carried out at 1-2°C and a constant voltage of 40 V.
The pore widening process is performed after the end of the anodic oxidation by immersion of the samples in a 0.86 M phosphoric acid aqueous solution. This process results in partial dissolution of the pore inner wall surface and in the dissolution of the inverted barrier layer at the base of each pore.
In order to improve long range pore ordering of the PAA film, a two-step anodization process was applied in all cases. This process starts with a thick Al film, and part of it is consumed by anodization and alumina dissolution. Pore initiation sites for the second anodization step are thus formed, which help obtain perfect long range pore ordering of the PAA film.
Pattern transfer to the Si substrate
Nanopatterning of Si through self-assembled porous anodic aluminum oxide thin films is an interesting lithography-free process for fabricating regular nanoscale patterns on the Si wafer. The area to be patterned can be pre-selected by patterning the Al thin film, which is then anodized using the appropriate conditions. Different processes were reported in the literature for pattern transfer through a PAA film; however, no systematic study was performed to achieve optimized pattern transfer to the Si wafer. Reported works include electrochemical etching of Si through the PAA film [1, 3], electrochemical oxidation of Si through the PAA pores, followed by the removal of the PAA film and wet chemical etching of the remaining undulated electrochemical SiO2 layer [18, 19], and reactive ion etching of Si through the PAA mask using SF6 gas or a mixture of CF4:Ar:O2 gases [20, 21]. In most of the above, the patterned features on the Si wafer were very shallow, and the pattern transfer anisotropy was not considered.
In this work, we systematically investigated the etching of Si through a PAA masking layer directly developed on the Si wafer by anodic Al film oxidation. We used reactive ion etching based on fluorine chemistry and, more specifically, we used three different gases: SF6, a mixture of SF6 and O2, and a mixture of SF6 and CHF3. For these different gases, we examined the etch rate and pattern transfer anisotropy to get all parameters for obtaining the designed pattern.
PAA mask formation
Characteristics of the PAA layers in the three different samples used in this work
PAA thickness (nm)
Pore size in nm after pore widening for 40 min
35 – 45
35 – 55
35 – 45
Reactive ion etching of Si through the PAA mask
The mechanisms involved in reactive ion etching combine physical (sputtering) and chemical etching. The gases or mixture of gases used and the RIE power and gas pressure are critical parameters that determine the etch rate. The etch rate is also different on large Si surface areas compared to the etch rate through a mask with nanometric openings. In this work, the PAA mask used showed hexagonally arranged pores with size in the range of 30 to 50 nm and interpore distance around 30 nm.
Three different gases or gas mixtures were used: SF6 (25 sccm), a mixture of SF6/O2 (25 sccm/2.8 sccm), and a mixture of SF6/CHF3 (25 sccm/37.5 sccm). In the first case, the etching of Si is known to be isotropic, while in the last two cases, it is more or less anisotropic. Separate experiments were performed for each gas mixture. In all cases, we used three different etching times, namely, 20, 40, and 60 s. The conditions used for the RIE were as follows: power 400 W and gas pressure 10 mTorr.
Effect of Al annealing before anodization
Results and discussion
Under the plasma conditions used, the etch rate in SF6 gas measured on large patterned areas (100 × 100 μm2) is approximately 700 nm/min and etching is isotropic. In the case of etching through the PAA mask, the etch rate was found to be much lower (in the range of 140 to 180 nm/min). This etch rate reduction is expected and is due to the small diameter of the alumina pores (this effect is known as ‘etch lag’).
Etch rate of Si through an Al mask compared to a SiO 2 mask with large openings
Large area Si etch rate (nm/min)
Etch rate through the PAA mask(pore diameter in the range of 35 to 45 nm) nm/min
140 – 180
140 – 180
65 – 85
With SF6, the etch rate is drastically reduced through the PAA mask compared with the large area etch rate. However, the addition of oxygen in SF6 does not create any significant difference in the etch rate compared with SF6, as in the case of large area etching. The only effect is a slightly better anisotropy. The significant difference is between these two gases and SF6/CHF3. In this last case, the etch rate is lower, and better anisotropy is achieved compared to the first two cases.
In general, the mixture SF6/CHF3 gives highly anisotropic Si etching. This is due to the fact that with the addition of CHF3 to the SF6 gas, CF2 radicals are produced that form a C x F y blocking layer on the Si sidewalls during etching . This thin fluorocarbon polymer limits the rate at which fluorine radicals from the plasma reach the Si surface. In addition, it limits the rate of diffusion of volatile SiF y species into Si and, therefore, slows down the chemical etching. Concerning the etch rate in SF6/CHF3, it is lower compared with both SF6 and SF6/O2 gases. This is due to the fact that the F-atom density is barely higher in this mixture compared to the two other cases, thus retarding Si etching .
In Table 2, a comparison is made between the etch rate of a 100 × 100 μm2 Si area formed using a resist mask and the etch rate of Si through the PAA mask (pore diameter in the range of 35 to 45 nm). The thickness of the PAA mask was 400 nm. Several samples were considered, and the range of given values is an average of all measured values. As described above, the etch rate is similar with SF6 and SF6/O2, while it is lower with SF6/CHF3. By increasing the PAA mask thickness from 400 to 500 nm, the etch rate in SF6/CHF3 was reduced from approximately 70 to 50 nm/min.
Feature etch depth using SF 6 /CHF 3
PAA layer thickness (nm)
Etching time (s)
20 nm (lower due to partially etched walls)
We investigated in detail the RIE of Si through a PAA mask for surface nanopatterning using SF6, SF6/O2, and SF6/CHF3 gases/gas mixtures. It was found that in all cases, the etch rate through the PAA mask was significantly lower than that on non-masked areas. The smallest etch rate and best anisotropic profiles were obtained with the SF6/CHF3 gas mixture. Using a PAA mask with highly ordered hexagonally arranged nanopores, a perfect pattern transfer of the nanopores to a large Si area is achieved. The same is possible on small pre-defined areas on the Si wafer.
VG and AO are post-doctoral researchers. AGN is the director of research at NCSR Demokritos/IMEL and the head of the “Nanostructures for Nanoelectronics, Photonics and Sensors” research group.
This work was partially financed by the 03ED375 PENED research project with funds from the Greek Ministry of Development (80%) and EU (20%). Funding was also received from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement NANOFUNCTION n°257375.
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