Amorphous carbon films have advantageous properties as resist mask for lithography, e.g. the stability against halogen plasma etching, negligible chemical reactivity with most substrates and the possibility of removing the mask by reactive ion etching with oxygen (Bouchiat and Esteve, 1996) after patterning the metal film. Recently we have shown (Mühl et al., 1997), that small trenches in amorphous carbon films can be produced by a field induced local oxidation with a voltage biased tip in an scanning force microscope (SFM). The depth of the holes and the trenches corresponds to the total thickness of the carbon film while the width was found to be as small as about 20 nm. In contrast to this, the field-induced oxidation of silicon (Pérez-Murano et al., 1995) is blocked after a few nanometers by the formation of the isolating silicon oxide. We now have additionally demonstrated the transfer of the carbon patterns into metal films using argon ion beam etching. By this new method we produced 15-nm-trenches in AuPd films. Results for attempts to produce similar gaps in narrow AuPd lines will be discussed. In order to control and minimize the width of the gaps, the resistance of the conducting lines was controlled in situ during ion beam etching. Possible applications of these small patterns are in the field of single electron devices and molecular electronics.
Scanning probe microscopy is developed to an active tool for nano-patterning of surfaces and thin films. In particular, SFM's were used for exposing e-beam-resists (Ishibashi et al., 1998) or engraving in soft layers (Bouchiat and Esteve, 1996) or metals (Rank et al., 1997). Another possibility is the electric field induced local oxidation underneath a cantilever tip in an SFM which was demonstrated on crystalline and amorphous silicon (Snow et al., 1996). These oxide patterns can serve as etch masks. This technique was used for the laboratory production of single semiconductor devices (Minne et al., 1995).
In the literature, amorphous carbon (a-C) is often discussed as material for etch masks, because it has a lot of advantageous properties, like the negligible chemical reactivity with most substrates, the stability against halogen plasma and the possibility of removing the carbon by oxygen or hydrogen reactive ion etching. Patterns which are generated by a-C masks often show quite smooth side-walls and edges and are, therefore, interesting for applications in optoelectronics. The a-C is often incorporated as the bottom layer of a two-layer resist system (Kragler et al., 1995). After patterning of the top layer, the structure can be transferred into the carbon layer in a second step.
The direct patterning of a-C is also possible by oxidation with an electron beam in an oxygen atmosphere. Minimum features of about 500 nm in width were obtained with a scanning electron microscope (Wang et al., 1995). Recently, we have demonstrated that SFM based field-induced local oxidation of a-C layers can produce patterns with widths well below 100 nm (Mühl et al., 1997). It was shown that there is a threshold voltage for the oxidation process. The patterned holes or trenches do not need any additional developing or etching steps. The oxidation depth of the a-C, and hence the depth of the patterns themselves, are not limited by passivation processes due to the volatile oxidation products. Thus it is possible to directly produce lithographic a-C masks in a one step process in the SFM and to transfer the pattern into underlying metal layers by ion etching.
Materials and Methods
The procedure for the field induced oxidation of the a-C films in an SFM is already described elsewhere (Mühl et al. 1997). The 15 nm thick a-C films are prepared by rf-magnetron sputtering from a graphitic carbon target on top of a 20 nm thick Au80Pd20 alloy. The mean crystallite size of the AuPd films depends on the deposition technique and varies from about 5 nm for thermal evaporation to around 40 nm for sputter deposition. The substrates are oxidized polished silicon wafers.
The a-C mask patterns were transferred into the AuPd films by Ar ion dry etching. The operating conditions were 6x10-4 mbar Ar pressure, an Ar-ion energy of 800 eV and a current density of 0.2 mA/cm2, resulting in etching rates of 0.17 nm/s for Au-Pd and 0.01 nm/s for the a-C resist material. The patterns were imaged mainly with the SFM both after the field induced oxidation of the a-C films as well as after the transfer of the patterns into the metal. Here, we would like to stress that the tips used for patterning the a-C are not damaged by this prodecure. It is therefore possible to use the same tip for the four main steps involved, i.e. imaging and patterning the a-C film and imaging the patterns in the a-C mask and the metal film. In order to check the SFM results and to get a feeling for the image distortion by the shape of the tip, scanning electron microscopy (SEM) was additionally used for imaging the patterns.
Carbon Patterning by SFM
In Fig. 1, we show an SFM-image of four trenches produced by field induced oxidation of a 15 nm thick a-C film. In contrast to our previous publication, we now used a combined constant current - constant force mode, i.e. during patterning both the deflection of the cantilever and the current flowing from the SFM-tip to the a-C film were held constant. This was done by varying the sample height in the conventional constant force mode of the SFM for the cantilever deflection and by simultaneously adjusting the voltage applied between tip and sample in order to obtain a constant current. For the trenches shown in Fig. 1, the current was set to 50 pA, 80 pA, 150 pA and 300 pA, respectively. During the patterning the voltage varied between 7 V and 12 V with a tendency to larger voltages for larger patterning currents. Each of the lines was scanned five times in each direction with a speed of 1 µm/s.
Figure 1: An SFM topography of the a-C film after the field induced oxidation of four lines with increasing exposure. The size of the image is 700 nm x 700 nm. The smallest trench on the left size has an apparent width of around 20 nm.
Pattern Transfer into Metal Films
In Fig. 2, we show the result of using the a-C pattern of Fig. 1 as a resist mask for patterning the underlying AuPd film with Ar ions.
Figure 2: SEM image of the AuPd film after the pattern transfer from the a-C mask. The mask material is already removed by reactive ion etching with oxygen.
Both the produced trenches as well as the grain boundaries of the AuPd film are clearly visible in Fig. 2. Comparing the widths of the trenches in the a-C film of Fig. 1 and the etched trenches in the AuPd of Fig. 2, a good transfer of the mask geometry into the metal can be stated. The smallest trench width obtained by so far amounts to about 16 nm.
It seems, however, to be of considerably larger interest, to produce gaps in conducting lines instead of trenches in films. We, therefore, covered a AuPd line prepared by conventional e-beam lithography with a a-C film and subsequently produced a trench across the underlying metal. Following the same steps as already discussed, we obtained the result shown in Fig. 3.
Figure 3: A gap in a 200 nm wide AuPd line produced by an SFM defined trench in an overlying a-C film and subsequent Ar ion etching. The gap width amounts to about 20 nm. There is still metallic conduction across the gap.
This result demonstrates the possibilty of obtaining both trenches as well as gaps in metal films and conducting lines with minmum feature sizes smaller than 20 nm. It turns out, however, that the Ar ion etching process used to transfer the a-C pattern in the metal is very critical especially for the preparation of small gaps. The gap shown in Fig.3 has, e.g. still metallic conductance, pointing to slightly inhomogeneous Ar ion etching. This might be caused by a decreased thickness of the a-C film in the vicinity of the trench, which is basically an effect of the finite radius of curvature of the Si SFM-tip.
Discussion and Outlook
We have shown the possibility of preparing nm-sized trenches and gaps in metal films and conducting lines with a new method combining field induced local oxidation of an a-C film in an SFM with conventional e-beam lithography and Ar ion etching processes. The minimum feature sizes obtained so far are in the range between 15 nm and 20 nm. These patterns could be already suitable for the production of single electron (SET) devices with enhanced operation temperature and for molecular electronics. We will concentrate especially on the SET devices and on the improvement of the Ar ion etching procedure. In particular, a combination of the Ar ion etching with an in situ control of the line resistance seems to be a promising method for the minimization of the gap width. First results point to obtainable widths well below 10 nm.
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Prof. Guenter Reiss
Experimental Physics, University of Bielefeld, Fac. Physics
Universitätsstrasse 25, 33615 Bielefeld, Germany
Tel.: +49-521-106-5428, Fax: +49-521-106-6046