Dynamic Shadow Mask Technique: A Universal Tool for

be exploited in a positive way to produce interesting patterns and low dimensional objects, such as thin wires along step edges6 (for a review see...
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NANO LETTERS

Dynamic Shadow Mask Technique: A Universal Tool for Nanoscience

2005 Vol. 5, No. 1 15-20

Stefan Egger,*,† Adelina Ilie, Yiton Fu, Jeffrey Chongsathien, Dae-Joon Kang, and Mark E. Welland* Nanoscience, UniVersity of Cambridge, 11. J J Thomson AVenue, Cambridge CB3 0FF, England Received August 14, 2004; Revised Manuscript Received October 16, 2004

ABSTRACT A comprehensive instrument, designed for fabricating nanostructures by evaporation through a dynamic shadow mask in ultrahigh vacuum, is described. The versatility and performance of the instrument is demonstrated through a series of examples, allowing for applications that are impossible to achieve with traditional nanopatterning methods. Clean nanostructures or entire devices made of different materials and on various substrates can be fabricated. The technique is compatible with fundamental surface science and can be easily interfaced with other fabrication and characterization techniques.

Structures that are small enough to show size dependent new properties are very sensitive to contamination, which can affect their behavior. For such structures, a clean fabrication process is crucial. Evaporation through a shadow mask in ultrahigh vacuum satisfies this requirement as it avoids contamination by resist, chemicals, or exposure to air. This technique has been demonstrated on structures for fundamental surface science research, with various sizes from the submicrometer range1,2 down to 10 nanometers,3 and various shapes, such as thin magnetic structures and multilayers,2,4 or could allow fabrication of self-organized clusters,5 atomic wires,6 or more complex patterns. It can also be used for curved surfaces,3 and, as shown here, on very fragile samples. When used in a dynamic mode, where the motion of the mask over the surface ‘draws’ the nanostructures, as described in this paper, a combination of different materials and shapes is straightforwardly fabricated. Additionally, since the structure thickness can be varied freely and continuously, three-dimensional patterning of nanostructures is possible. Previous realizations of the technique, either with a fixed mask/sample arrangement2,7 or in dynamic1,3,8,9 mode, have already shown impressive results. However, in these cases the range of technical capabilities and applications was rather limited as the instruments were designed for specific tasks. With further development we believe the technique can achieve its full potential: to provide a universal tool for nanoscience. In this letter, we describe our developments toward this end, and show examples of fabricated nanostructures that illustrate its enhanced technical performance and * Corresponding authors. Stefan Egger, E-mail: [email protected] and Mark Welland, E-mail: [email protected]. † Currently at National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 10.1021/nl0486822 CCC: $30.25 Published on Web 11/25/2004

© 2005 American Chemical Society

the larger range of applications that we can achieve. We have focused on applications for research or prototyping, though the technique could in principle be used for high throughput fabrication8,10 due to its capability to produce structures in parallel. Figure 1 gives a schematic overview of the instrument described here. Its key parts and characteristics can be summarized as follows. (i) Structures grown in a UHV environment, and capability to prepare both substrate and mask independently in-situ (for example by annealing); (ii) Multiple evaporation sources allowing combination of different materials; (iii) Stencil mask-cantilever with custommade holes or patterns; (iv) Precise and stable lateral positioning, down to several nanometers, of the mask relative to the sample; (v) Small and uniform gap between mask and sample over the entire sample area, achieved through electronic approach and alignment of the mask; (vi) Software for user interface and automatic generation of patterns; (vii) Interfacing with instruments for in-situ characterization, such as atomic force imaging with commercial cantilevers. In progress: imaging with the stencil mask-cantilever for fabricating structures aligned relative to preexisting structures. We now give a more detailed description of each of the parts. The UHV system consists of three chambers (main chamber, evaporator chamber, and load lock), separated by gate valves. All parts can be baked at 150 °C. The base pressure is better than 1 × 10-9 mbar and rises to the 10-9 mbar range during evaporation. To reduce mechanical noise, the system is mounted on vibration dampers and only ion pumps are used during structure fabrication or imaging. The sample surface faces downward while the evaporator sits in a vertical position, 20 cm below the sample, in a

Figure 1. (a) Schematic diagram of the instrument: (1) sample mounted on the approach and alignment assembly, composed of inchworm, flexure stage, and two inertial sliders (x-y and tilt); (2) stencil mask cantilever; (3) e-beam evaporator; (4) laser; (5) quadrant photodiode; (6) light source; (7) optical microscope with CCD camera. (b) Examples of patterns cut in the silicon nitride membrane of the mask. (c) Pt tip defined on the stencil mask by FIB. (d) Line scan showing 10 nm sharpness of Cu features evaporated on oxidized Si substrate (dimensions are in nm). A commercial AFM cantilever was used for scanning in situ in noncontact mode.

separately pumped chamber. A commercial e-beam evaporator with four pockets is used, with additional homemade resistive heated crucibles for organic materials, such as C60. The evaporation rate is typically in the range of nanometers per minute, depending on the material. The stencil mask cantilevers were made from silicon wafers with a low stress silicon nitride coating, from which silicon nitride membranes 100 to 400 nm thick were defined using photolithographic and etching processes. The active area of such a membrane is about 1-2 mm2, in which holes and patterns were cut using a focused ion beam (FIB) microscope (Figure 1b). With FIB, the minimum hole size can be less than 40 nm, this size being limited by the minimum thickness of the silicon nitride membrane (about 100 nm), which can be reliably used for our large size stencils.11 Smaller feature sizes can be defined in thinner membranes and by using subsequent coating to achieve controlled closure of the holes,12 or through a more complex process involving e-beam lithography.3,13 However, a mask with larger feature size has a longer lifetime, as closure of the holes via clogging by evaporated material takes a longer time. Circular holes close after evaporating a thickness equivalent to about 1.5-5 times the hole diameter,7,9,14 depending on the evaporated material, mask material, and beam collimation. Coating with alkyl self-assembled monolayers has been proved to minimize the clogging,14 and generalization of the method would greatly increase the throughput of the technique and would allow it to be scaled up. For dense and complex patterning, the technique’s efficiency can be further increased by using microfabricated shutters to independently open and close apertures.15 A tip can also be defined on the silicon frame of the stencil mask for the purposes of scanning and alignment (Figure 1c), as detailed below. For some applications, we can also readily employ commercial TEM silicon nitride membranes and mesoporous membranes as well as larger, micron-size stencils made in solid metal. The stencil mask can be moved laterally and positioned with nanometer precision relative to the sample using a 16

commercial UHV compatible flexure stage (Physik Instrumente, Germany). This stage has a capacitive sensor to monitor and regulate the position with a resolution of 1 nm and repeatability of ( 2.5 nm over its full motion range of 100 × 100 µm2. Consequently, piezo drift and creep can be eliminated, unlike the case of open-loop nanopositioning systems. In some situations, such as interconnecting nanoobjects or fabricating complex structures through the combined use of several masks, it is desirable to extend the motion range over an area much larger than that offered by the flexure stage. To this end, we developed an inertial slider so that the overall lateral range now extends to about 5 × 5 mm2. An optical microscope with CCD camera is used for coarse positioning. To allow fabrication of small structures using shadowmask evaporation, it is crucial that the gap between the mask and the sample a be as small as possible, typically less than 1 µm. The minimum achievable size s of grown nanostructures is then the sum of the minimum feature size that can be defined in the stencil mask sm, plus a boundary zone sb, whose extent is determined only by the geometry of the evaporator mask sample configuration if diffusional processes are negligible (see below): s ) sm + sb ) sm +

aD b

(1)

Here, D is the diameter of the evaporator output, b the evaporator-to-mask distance, and a the mask-to-sample distance, so that with typical values D ≈ 2 mm, b ≈ 20 cm, a ≈ 1 µm, sb ≈ 10 nm is obtained (see Figure 1d). Further decrease of the feature size can be obtained by adjusting the geometry according to eq 1. Because in this paper we choose to focus on aspects that are unique to the dynamic shadow mask technique, rather than being concerned with the minimum achievable structure size, the stencil masks have been fabricated with FIB and without any subsequent procedure for size reduction. Consequently, the sizes of our Nano Lett., Vol. 5, No. 1, 2005

demonstration structures are in the 80-500 nm range, though 10 nm size nanostructures have been achieved with the technique.3 The mask-to-sample gap must also be uniform over the whole mask area, while the mask should not touch the sample in an uncontrolled way. Technical solutions for linear approach with nanometer precision are well-known from scanning probe microscopy. An inchworm and a piezo tube were thus used for coarse approach and fine adjustment, respectively. To ensure a homogeneous gap over the whole mask area, a second inertial slider stage has been developed, which tilts the whole sample-flexure stage-inchworm assembly inside the UHV system. This design was very challenging technically as the flexure stage is rather heavy (about 1.3 kg) and has several electrical connections which, being very sensitive to noise, require coaxial shielding. Consequently, the design had to be very stiff and able to compensate the forces introduced by the connecting cables. The tilting stage has a range of 2 degrees in both angular directions, with a step size of 5 µrad. A crucial point is to adjust the relative alignment between the mask and the sample in vacuum, and for this a laser beam deflection technique is used. Accordingly, a laser beam is reflected first by the sample and the reflection position far away (about 2 m) from the mask is recorded; then, the sample is replaced by the mask, and the new beam reflection is recorded relative to the previous one. The angle between the two reflected beams is then compensated using the tilting stage. By using a quadrant photodiode for beam detection, a precision of several µrad can be achieved easily. To ensure that even complex structures are fabricated in a convenient and controlled way, both mask approach and positioning and control of the evaporation process are performed fully automatically through dedicated software. When using different materials, the evaporator can be moved by a stepper motor so that the active crucible is always brought to the same position. In this way, misalignment of the subsequent levels of evaporated structures due to parallax effect is avoided. Adding the capability of scanning probe imaging and characterization to the instrument is an important step toward developing it into a universal tool for nanoscience. Currently, fabricated structures can be imaged in noncontact AFM mode by replacing the mask with a conventional AFM cantilever (Figure 1d). Imaging with the same stencil mask that is used for structure fabrication has the enormous advantage that it allows one to fabricate new structures aligned in the desired way relative to other structures (such as nanotubes and nanowires, DNA strands, etc.) positioned or grown on the substrate a priori by other techniques. This step is currently under development and requires defining a tip on the stencil mask, either by Pt deposition using FIB (as shown in Figure 1c), or by direct catalytic growth. The biggest difficulty that we are working to overcome is scanning stability when using the large and very stiff stencil masks. In an earlier step of development of the technique, both stencilling and scanning could readily be achieved with a commercial Si cantilever in which a limited number of holes have been drilled to Nano Lett., Vol. 5, No. 1, 2005

Figure 2. Structures with thickness modulation. (a) AFM image of 8 µm long Co lines on silicon dioxide/silicon substrate with thickness modulating between 4 and 25 nm. An equivalent thickness of 270 nm of Co was evaporated. (b) SEM image showing how movement in one direction of a mask composed of five adjacent squares produces thickness modulation in two directions, along and perpendicular to the direction of motion. The mask was moved with increasing speed from top right to bottom left, and finally kept in a fixed position for 10 min. Width and thickness of the squares: 400 and 15 nm, respectively; material: C60 on SiO2/Si. (c) AFM image of an “anti-dot” structure obtained by circularly moving a mask with an array of 1 µm diameter holes with 1:1 empty-to-fill ratio.

define the stencil.8 However, this is not a viable solution for many applications, given the highly reduced dimensions of this mask (practically defined by the size of the cantilever) compared to the purpose-designed silicon-nitride masks described above. To illustrate the performance of the instrument and possible applications, several categories of structures have been fabricated. Figure 2a-c shows examples of structures with thickness modulation. This capability is a notable advantage of our technique over common lithographic methods, and even over focused ion beam techniques (commercial or dedicated, such 17

as “ion-beam sculpting”16), which could in principle arbitrarily shape structures but with the drawback of introducing contamination and damage. In principle, any thickness modulation could be obtained by scanning a mask with a single hole over the sample, with variable speed. However, in general, structure fabrication combines both serial and parallel aspects.1,8 The resulting structure has a threedimensional shape obtained through convolution between the mask geometry and the motion of the mask: A(x,y) ) C

∫ T(x′,y′)M(x - x′, y - y′) dx′ dy′

(2)

Here M is the thickness profile of the structure grown without moving the mask, which is determined by the shape of the pattern cut in the mask and boundary tempering due to half shadow effects, T is proportional to the time the mask remains in a certain position (thus, inversely proportional to the moving speed), while C is a constant depending on the incoming material flux during evaporation. Each of the modulated lines shown in Figure 2a was fabricated by moving a mask with three circular holes spaced in a row, with speed oscillating according to y ) c/(1 0.75cos(2πyλ)) (with c ) 1 µm/h and λ ) 850 nm), where y is the lateral displacement. To increase the fabrication speed, a more sophisticated mask design can be used, with the cut pattern adapted as much as possible to the desired structure shape. Figure 2b shows a more complex structure illustrating this point, where the thickness varies in two directions, wedge and zigzag, though the mask has been moved in only one direction (here the pattern cut in the mask consisted of five adjacent squares). Structures of this type, with two-dimensional modulations in either micron or submicron ranges, can be ideally suited to optical applications to control the photonic band gap of surface plasmons.17 Figure 2c shows an “anti-dot” type modulated structure, obtained by circularly moving a mask with an array of holes with large coverage ratio. Figure 3a,b shows structures obtained by combining different materials. Multilevel patterning in the nanometer range is easily achieved by stencilling, while it is very difficult and impractical in the case of e-beam lithography. Figure 3a shows Cu lines aligned to C60 rings, obtained by a succession of circular and translational motions. This structure also illustrates the capability of our technique to produce small size patterns of soft materials, such as C60, that are impossible to pattern by lithographic methods. Figure 3b shows a C60 and Ni structure obtained by using two different masks, demonstrating registration and alignment at the nanometer scale. Another specific application of the shadow mask technique is to allow formation of nanostructures by controlling material diffusion on specially selected substrates. Though uncontrolled diffusion could increase the size of the nanostructures in an undesired way, above the value given by eq 1, this phenomenon could also be exploited in a positive way to produce interesting patterns and low dimensional objects, such as thin wires along step edges6 (for a review see refs 18-20). Restricting the lateral extent of the incoming 18

Figure 3. Combination of different materials (SEM images). (a) 80 nm wide Cu lines centered on 100 nm wide C60 rings obtained by moving a mask with circular holes, first circularly and then translating it. (b) Letters having C60 (black) and Ni (white) parts obtained by “stitching” the two masks shown in the upper part of the image.

Figure 4. C60 structures obtained through diffusion on PdAu/Cr/ Si(100) substrate (SEM image). The thickness of the thicker central region is 5 nm, while the boundary region formed through diffusion is a monolayer thick, as checked by AFM. A 15 nm nanogap was formed between two adjacent such structures.

material through the nanostencil mask would allow more control and potentially produce different types of nanostructures than would be achievable with homogeneous evaporation. In applications where diffusion has to be minimized, such as interconnecting wires, Er7 or Co on oxidized Si appears suitable. Conversely, there are also cases where the mobility of atoms can increase the sharpness of structures, such as Ag on Si(111)-4 × 1-In,21 to generate almost atomically sharp boundaries. Figure 4 shows two oval C60 structures, evaporated on PdAu film deposited on a Si substrate (after deposition of a 3 nm thick Cr adhesion layer) by moving a mask with a single hole. In this case, there is increased diffusion of C60 on PdAu, coupled with the general phenomenon of C60 self-diffusion on underlying C60 layers. For each individual structure, the overall diffusion led to the formation of two distinctive zones, a central, thick zone, and a boundary zone, about 30 nm wide, much thinner than the central one and with uniform thickness. In addition, a Nano Lett., Vol. 5, No. 1, 2005

Figure 5. Interconnection of a nanotube rope without lithography (Cu pattern on mica substrate), from (a) micro- to (b) nanoscale obtained by two successive levels of stencilling, using micro- and nanoscale masks, respectively. Panels (a) and (b) are optical images, (c) is an AFM image.

nanogap of about 15 nm was formed between the two structures, which could, in principle, be continuously varied to a smaller value. Diffusion is highly dependent on temperature and can be enhanced by increased humidity (when in air) or by the condition of the surface (pristine or with residual contaminants), and this fact can affect the temporal stability of nanostructures. Consequently, stability of fragile structures (such as those made of C60) can be significantly increased by storing the sample at relatively low temperature and in vacuum. Alternatively, stability can be achieved by the addition of a capping layer, such as Ge or Al2O3. On the other hand, other combinations of materials are found to be very stable in ambient conditions, such as cobalt on silicon covered by silicon dioxide. Nanostructures with a thickness as low as 4 nm and lateral size of about 100 nm were found to conserve their shape and lateral size even after a year of storage in air, though an oxide layer is expected to have been formed. Figures 5, 6, and 7 show examples of interconnecting onedimensional nanoobjects. In Figure 5 a nanotube rope has been connected to the outside world without any lithography, just by using two levels of stencilling, one at the micron scale employing a stencil made out of solid metal, and another one at the nanometer scale, using the usual silicon nitride stencil masks. Combining different masks, as here, has the advantage over using just a single micro-to-nano mask, as in ref 10, in that it allows changing materials and varying the thickness in various regions of the pattern (as shown in Figure 6). Figure 6 shows a similar lithography-free interconnection procedure, this time performed on a fragile substrate, in this case a silicon nitride membrane. This on-membrane device has been developed so that correlated physical (transport or scanning probe) measurements and HRTEM can be performed on individual one-dimensional nanocomposites, and for this reason the nanocomposite is suspended over thin Nano Lett., Vol. 5, No. 1, 2005

Figure 6. A full device on a 70 nm thick, 0.5 × 0.5 µm2 silicon nitride membrane supported by a silicon frame for correlated physical property measurements and HRTEM on suspended onedimensional nanocomposites. Two levels of stencilling were used at the micro- scale, and one at the nano- scale. (a) Optical image showing the interconnect lines and bonding pads, with different thicknesses and materials, 15 nm thick PdAu/Cr and 500 nm PdAu/ Cu/Cr, respectively. (b) AFM image of a suspended nanocomposite crossing a 50 nm wide slit pre-cut in the membrane prior to electrical contact fabrication. (c) I-V and dI/dV curves showing level quantization consistent with a probed length of nanocomposite equal to the slit width.

(down to 50 nm) slits between the electrodes (as shown in Figure 6b). This is a case that made use of both purposedesigned inertial sliders, as it required lateral alignment relative to the preexistent slits on the nanometer scale, and angular alignment to achieve a uniform stencil-sample gap so that several connections could be made covering a large area, of about 0.5 mm2. Care was taken to avoid closing of the slits of the suspended structures when making the electrical interconnects. Figure 6c shows an I-V measurement taken in such a configuration, which, due to the short length of nanocomposite that is probed, indicates quantization effects.22 Finally, Figure 7 shows a procedure we developed to be used in conjunction with scanning probe microscopy in UHV (that requires maintaining a clean surface) on randomly dispersed one-dimensional structures (in this example, carbon nanotubes). The principle is to provide a common contact for as many one-dimensional structures as possible, while a conductive AFM tip would provide a second, individual 19

Research Collaboration (IRC) in Nanotechnology funded by EPSRC. One author (S.E.) acknowledges support from the Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. References

Figure 7. Percolation path between individual semicircles (PdAu/ Cr), defining a common electrical contact over an area of several mm2 for carbon nanotubes randomly dispersed on an insulating substrate (SiO2/Si). The nanotubes are not entirely monodispersed due to the use of an organic solvent, rather than a surfactant, to remain compatible with UHV requirements. The mask was a Millipore membrane with 400 nm size circular pores, roughly uniformly distributed. Conductivity measurements along nanotubes were performed using a conductive AFM tip as a second contact. The inset shows I-V curves obtained in such a configuration (continuous line), and by directly probing the insulating substrate (dashed line).

contact in a conductivity measurement. Here, an economical mesoporous membrane has been used as a mask and moved in such a way that a percolation path is obtained over an area of several mm2 for the individual semicircles that form the common contact. I-V curves have been taken by contacting the other end of a nanotube with the AFM tip, and over the silicon-dioxide substrate. Comparison between the two curves shows that the current indeed passes along the contacted length of the tube and not through the underlying substrate. In conclusion, we have described developments of a novel nanofabrication technique based on dynamic nanostencilling through a shadow mask. This technique has broad applicability as both nanofabrication facility and research instrument for surface science, and therefore a great potential to be transformed into a universal tool for nanoscience. Acknowledgment. We thank Dr. Siang Huei Leong for his help with the SPM software. This work was supported by the European Union in the fifth framework program project ATOMS (IST-1999-14912) and the Interdisciplinary

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NL0486822

Nano Lett., Vol. 5, No. 1, 2005