Novel Self-shrinking Mask for Sub-3 nm Pattern Fabrication

It is very difficult to realize sub-3 nm patterns using conventional lithography for next-generation high-performance nanosensing, photonic, and computing devices. Here we propose a completely original and novel concept, termed self-shrinking dielectric mask (SDM), to fabricate sub-3 nm patterns. Instead of focusing the electron and ion beams or light to an extreme scale, the SDM method relies on a hard dielectric mask which shrinks the critical dimension of nanopatterns during the ion irradiation. Based on the SDM method, a linewidth as low as 2.1 nm was achieved along with a high aspect ratio in the sub-10 nm scale. In addition, numerous patterns with assorted shapes can be fabricated simultaneously using the SDM technique, exhibiting a much higher throughput than conventional ion beam lithography. Therefore, the SDM method can be widely applied in the fields which need extreme nanoscale fabrication.

It is very difficult to realize sub-3 nm patterns using conventional lithography for next-generation high-performance nanosensing, photonic, and computing devices. Here we propose a completely original and novel concept, termed self-shrinking dielectric mask (SDM), to fabricate sub-3 nm patterns. Instead of focusing the electron and ion beams or light to an extreme scale, the SDM method relies on a hard dielectric mask which shrinks the critical dimension of nanopatterns during the ion irradiation. Based on the SDM method, a linewidth as low as 2.1 nm was achieved along with a high aspect ratio in the sub-10 nm scale. In addition, numerous patterns with assorted shapes can be fabricated simultaneously using the SDM technique, exhibiting a much higher throughput than conventional ion beam lithography. Therefore, the SDM method can be widely applied in the fields which need extreme nanoscale fabrication.
Because of the rapid evolution of nanosensors, nanophotonics, and semiconductor technology down to the sub-10 nm technology node, the research of sub-10 nm patterning has attracted much attention in recent years [1][2][3][4][5][6] . For instance, sub-10 nm electromagnetic hot spots with extreme near-field confinement are crucial to plasmonic applications [6][7][8] . Nanoscaled graphene, quantum point contacts, and quantum dot devices for high level computing require precise sub-10 nm patterning 9 . Thus a variety of approaches including the helium and neon ion beam lithography 10,11 , electron beam lithography [12][13][14][15][16] , advanced scanning probe lithography 17 , nanoimprint lithography 18,19 , and extreme ultraviolet lithography (EUV) [20][21][22] , have been proposed in order to resolve the difficulties of the sub-10 nm patterning. Nonetheless, these methods all have their individual problems which are difficult to be overcome. For example, ion beam, electron beam, and scanning probe lithography are capable of high-resolution patterning, but their throughput is typically low [23][24][25][26][27] . Although the high-voltage electron-beam lithography based on an aberration-corrected scanning transmission electron microscope (TEM) is a very promising approach to achieve the patterns down to 2 nm 15,16 , large-area patterning is not feasible currently using this method because a large sample cannot be put into the TEM holder. Nanoimprint lithography suffers from the surface sticking issue and poor mold condition 19 . As for the EUV lithography, although a short wavelength of 13.5 nm provides a high potential for nanopatterning, EUV lithography has not yet to be proven effective and needs further development due to the challenges from photoresist, mask, optical reflectivity, and source power 21,28,29 . Consequently, quadruple patterning is under high consideration for the 7 nm technology node in semiconductor industry due to the delay of the EUV lithography. However, how far the quadruple patterning can go depends on the cost, overlay control, and the cost-effectiveness of the extreme scaling 30 . Another technique such as ALD (atomic layer deposition) spacer technique has also been proposed to reduce pitch size 31 . However, the spacer is prone to collapse if the spacer thickness is less than 5 nm 31 . The shadow (angle) evaporation is also capable of obtaining sub-10 nm gaps based on the poor conformality of evaporation 32 . Nevertheless, it is not easy to precisely control the patterning by this approach.
It is difficult to achieve the patterns with a linewidth down to sub-3 nm scale using the conventional lithography based on focusing optical, electron, or ion beams. Hence, we conceive that instead of using focused beam approaches, shrinking the pattern itself will be a more practical and facile way to achieve the sub-3 nm fabrication. In this study, we demonstrate a new concept and method, entitled self-shrinking dielectric mask (SDM), to realize sub-3 nm patterns. The SDM method starts from an initial pattern on a hard dielectric mask, which could be defined by conventional optical, ion beam, or electron beam lithography. Afterwards the sample is exposed to the ion irradiation for a couple of minutes, and then the linewidth of the initial patterns on the mask will shrink during the ion irradiation. Together with a high aspect ratio, a critical dimension down to 2.1 nm has been realized with the SDM method in this study. Besides, a large area of nanopatterns can be fabricated Scientific RepoRts | 6:29625 | DOI: 10.1038/srep29625 simultaneously by the ion irradiation on SDM, instead of using sequential line-by-line writing such as those in the focused ion beam and electron beam lithography. Thus, a considerable amount of process time can be saved and the throughput can be greatly improved. Actually, one of the practical applications of SDM is shown in the supporting information. We have implemented the SDM method to define a sub-10 nm gate in the junctionless Si transistors on silicon-on-insulator substrate. Therefore, the SDM technique is an effective, high-throughput, and practically applicable nanofabrication technique to tailor the structures with the feature sizes down to a few nanometers, which can be applied not only in the high-performance computing devices, but also in the advanced nanosensors and nanoplasmonics. Accordingly, SDM shows a strong and promising potential to have a role in the next-generation high-throughput sub-3 nm fabrication. Figure 1 depicts the SDM method schematically. First, a hard dielectric layer such as Al 2 O 3 is deposited on the substrate (Fig. 1a), and then an initial pattern (e.g. a line array) is defined by conventional lithography (Fig. 1b). Afterwards, the sample is exposed to the ion irradiation for a couple of minutes, and the gap will shrink during the ion irradiation (Fig. 1c). The ion irradiation will be stopped once the target linewidth of nanogap is reached, and then a hard mask with the target linewidth is obtained (Fig. 1d). With this hard dielectric mask, we can transfer the patterns to the substrate by subsequent reactive ion etching or by further ion irradiation (Fig. 1e). Finally, the hard mask is removed away by etching after the patterns are transferred to the substrate (Fig. 1f).

Results
The scanning electron microscope (SEM) pictures of nanopatterns fabricated by SDM method are shown in Fig. 2. In Fig. 2a, an Al 2 O 3 line array with ~145 nm gap width was fabricated by focused Ga ion beam to act as the initial pattern for SDM. Figure 2b shows the cross section of the line array with a V-shaped geometry (a top width of ~145 nm and a bottom width of ~83 nm) which is mostly due to the redeposition effect 33 . This is hard to avoid when using focused Ga ion beam to fabricate deep nanopatterns. Afterwards, the Al 2 O 3 line array was exposed to the Ga ions, and the gap size started to shrink during the ion irradiation. After a couple of minutes, the gap shrunk to the sub-10 nm scale. Figure 2c shows an example of the cross section of the nanogap after the ion irradiation. The gap was estimated to be ~280 nm in depth with an opening of only 9.2 nm in width. This aspect ratio is ~30 which is quite extraordinary for sub-10 nm patterns 34 . Figure 2d illustrates a tiny gap size on the Al 2 O 3 mask of a critical dimension as low as 2.9 nm after the subsequent ion exposure. Further ion irradiation leads to a minimum gap width down to 2.1 nm as shown in Fig. 2e. This narrow linewidth is much smaller than the spot size of the focused Ga ion beam even with the lowest beam current (The smallest spot size is ~7 nm at the lowest beam current of ~1.1 pA for FEI Helios Nanolab 600i focused ion beam system). In fact, there is no need to focus the ion beam to get nanopatterrns using the SDM technique. The beam current we used in the experiment is as large as 2.5 nA, and the focused spot size at this beam current is about 133 nm 35 . Accordingly, with a 2.5 nA beam current, the optimal resolution for Ga ion beam lithography is not smaller than 133 nm. Nevertheless, a pattern down to 2.1 nm has been achieved by the SDM method, which clearly indicates that focusing the beam is not necessary to get a sub-3 nm pattern using SDM. Additionally, it is known that the ion milling rate is proportional to the beam current 36 . As a result, by using a more than two thousand times larger beam current (2.5 nA beam current rather than 1.1 pA), we have saved over 99.9% of time to fabricate sub-10 nm patterns. Finally, after the patterns were transferred to the substrate underneath the Al 2 O 3 layer, the Al 2 O 3 mask was removed by wet were transferred to the substrate by subsequent ion irradiation, revealing a critical dimension less than 10 nm (minimum linewidth is ~5.4 nm). It should be noticed that the patterns were transferred from the hard mask shown in Fig. 3f. It is seen that the linewidth of patterns transferred to the substrate (f) is comparable to that of the hard mask (Fig. 3f).
etching. An example of the sub-10 nm linewidth on the substrate is shown in Fig. 2f, demonstrating a linewidth as low as ~5.4 nm.
To observe the self-shrinking process more clearly, a series of SEM images were taken during the ion irradiation and are shown in Fig. 3. It is clearly observed that the gap size reduces dramatically from 145 nm to 5 nm with the increase of exposure time. The self-shrinking process shown in Fig. 3 also reveals that the pattern linewidth can be manipulated arbitrarily over a wide range from hundreds to a few nanometers, depending on the ion exposure time. In the supplementary information, Fig. S10 shows the Al 2 O 3 gap width as a function of the exposure ion dose, which were obtained from the SEM images taken after every dose increment of 55.86 pC/μ m 2 chronologically (shown in Figs S11~S13). It can be clearly seen from these SEM images that the line array is very uniform and nearly free of any defects across a wide horizontal field width (HFW). Figure S10 reveals a linear relation between the gap width reduction and the ion irradiation dose, with a wide range of the gap width from ~120 nm to ~5 nm. The result demonstrates that SDM is a highly controllable method for the precise fabrication of uniform nanopatterns.
Nanopatterns with a variety of different shapes are also feasible to be fabricated by SDM. In Fig. 4a, a circle ring array with a ~100 nm gap width as the initial pattern was fabricated by focused Ga ion beam. A gap width down to 4.3 nm was achieved by SDM as depicted in Fig. 4b. The inset in Fig. 4b is the cross section of the nanogap of the circle ring, revealing that the bottom width of the nanogap is only 2.7 nm. Apart from the circle ring The initial pattern with a gap width of 145 nm before the ion irradiation is shown in (a). After merely ~4 minutes of ion exposure, the gap linewidth shrunk to 75 nm (b). The images from (c-f) were taken chronologically after 6, 8.5, 9, and 10 minutes of the ion irradiation, respectively. The result clearly demonstrates the linewidth of the gap shrunk dramatically from 145 nm to 5 nm using the SDML method. array, initial patterns including a nanohole array with a diameter of ~120 nm and a square ring array with a gap width of ~100 nm were also prepared as shown in Fig. 4c,e, respectively. Figure 4d,f show the diameter of the nanoholes and the gap width of the square ring dramatically shrunk to 7 nm and 4.1 nm, respectively, after the ion irradiation. The result clearly demonstrates that the SDM method is capable of creating nanopatterns with different shapes of a critical dimension down to sub-10 nm scale. It should be noted that the surface of Al 2 O 3 inside the square become spherical (in Fig. 4e) after the ion irradiation of the square ring patterns.
The most attractive feature of SDM is the self-shrinking process of the hard dielectric mask. There might be a number of mechanisms behind this phenomenon. One of the possible explanation is the re-deposition effect during the ion beam irradiation. That is, the hard dielectric materials are sputtered from the base of the feature onto the sidewalls during the ion irradiation. It has been reported that the holes of the anodic aluminum oxide film were closed after the Ga ion beam irradiation, which was mainly ascribed to the re-deposition of the sidewall alumina to the top capping layer 37 . However, the re-deposition effect cannot explain the whole phenomenon we have observed. For example, Fig. 2b,c clearly shows that the Al 2 O 3 mask re-shapes from the wide V-shaped geometry to the straight nanogap with a high aspect ratio after the ion irradiation. It is not reasonable that re-deposition would form such a straight gap rather than a V-shaped geometry 33,38,39 . Furthermore, the spherical Al 2 O 3 surface as shown in Fig. 4e is unlikely to be solely caused by the re-deposition effect. As a result, re-deposition is not the only mechanism involved in the self-shrinking mask, and some other mechanisms should take place simultaneously. Another likelihood is the ion bombardment induced viscous flow and surface diffusion of the hard dielectric mask, which has been widely discussed in the previous studies 37, [40][41][42][43] . Please be noticed that the nanopattern as shown in Figs 2 and 3 is quite uniform and straight, which cannot be solely explained by either the re-deposition or the ion-induced viscous flow and surface diffusion. The charging effect might be the possible reason for this high quality nanopattern. When the sample is exposed to the ions, the non-conductive dielectric mask will be charged by positive ions 44 . The charging effect might keep the sputtered or flowing dielectric mask from being severely distorted due to the repulsive force between the positively charged mask. As a result, the line array could stay straight and uniform as shown in in Figs 2 and 3. The TEM image and energy dispersive spectroscopy (EDS) mapping of the nanogap cross section have been provided in the supplementary information. The EDS mapping indicates that the edge of the Al2O3 mask was highly doped by Ga, and so it can be deduced that the Al2O3 mask was charged during the ion irradiation. The TEM image shows that the Ga-doped Al2O3 was filled into the gaps, which might be attributed to the re-deposition or ion-induced viscous flow/surface diffusion. To sum up, the self-shrinking process may be a comprehensive manifestation of the above-mentioned mechanisms. It is noteworthy that the dielectric mask should be hard enough in order to prevent it from being milled out completely. If this happens, self-shrinking process will not occur. Therefore, Al2O3 was chosen as the mask in this study because of its extremely hard-wearing nature to ion milling.
Another attractive point of SDM is its capability to fabricate a myriad of nanopatterns simultaneously as long as the initial patterns are within the exposure area of the ion source. In Fig. 5, its ability to fabricate a large area of a uniform sub-10 nm line array is shown. A large exposure area of 3570 μ m 2 irradiated by the Ga ions is illustrated in Fig. 5a. In fact, even a larger area can be achieved by SDM as long as the initial pattern is within the exposure region of the ion source. Figure 5b depicts the uniformly fabricated sub-10 nm line array after the ion irradiation, and a small linewidth of only 5.1 nm is clearly manifested in Fig. 5c. Some particular methods had been proposed to define sub-10 nm patterns by focused ion beam, such as delineating patterns on a sample with a thick sacrificial metal layer using the lowest Ga ion beam current 45 , or utilizing helium or neon as the ion beam source 10,11 . However, these techniques are extremely time-consuming with a very low throughput to fabricate nanostructures within a large area, because of the low beam current (1.1pA) and the inefficient milling rate of helium and neon. For example, to fabricate a line array with a large area of 3570 μ m 2 (the same as that shown in Fig. 5a) with sequential line-by-line writing by focused Ga ion beam, it will take more than 610 hours using the lowest 1.1 pA beam current. Nevertheless, the minimum gap width is ~20 nm even using this lowest beam current. However, it takes only ~30 minutes (less than 20 minutes for the initial patterns and about 10 minutes for the ion irradiation) to fabricate a sub-10 nm (~5.1 nm) line array with a large area of 3570 μ m 2 using the SDM technique (Fig. 5). This result indicates that SDM is over 1000 times more rapid than the focused ion beam lithography and the feature size down to a few nanometer can be achieved at the same time.
In conclusion, we have proposed a novel concept to fabricate sub-3 nm patterns. Based on the self-shrinking mask instead of the focused beam approach, the SDM method can fabricate nanopatterns with the feature size of only a few nanometer, which is difficult to be fulfilled with the conventional lithography. We believe that the SDM technique opens a new door and impinges a great impact on the nanofabrication of sub-5 nm patterns in a variety of applications. Ion irradiation. The samples with initial patterns on the Al 2 O 3 mask was irradiated by Ga ions at 30 keV and a beam current of 2.5 nA (FEI Helios Nanolab 600i dual beam system). The samples were exposed to Ga ions with a field of view of 3570 μ m 2 for about 10 minutes to get sub-3 nm nanopatterns on the Al 2 O 3 mask. In situ observation was carried out by the SEM equipped in the dual beam system. Wet etching of the Al 2 O 3 mask. A l2 O 3 was etched away in dilute HF solution. Then the sample was observed by the SEM equipped in FEI Helios 600i dual beam system.