Organic Ice Resists - Nano Letters (ACS Publications)

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Organic ice resists William Tiddi, Anna Elsukova, Hoa Thanh Le, Pei Liu, Marco Beleggia, and Anpan Han Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04190 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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William Tiddi – Manuscript draft – Confidential 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Organic ice resists

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William Tiddi, Anna Elsukova, Hoa Thanh Le, Pei Liu, Marco Beleggia, Anpan Han*

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DTU Danchip/CEN, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

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Abstract

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Electron-beam lithography (EBL) is the backbone technology for patterning nanostructures and

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manufacturing nanodevices. It involves processing and handling synthetic resins in several steps, each

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requiring optimization and dedicated instrumentation in cleanroom environments. Here, we show that

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simple organic molecules, e.g. alcohols, condensed to form thin-films at low temperature demonstrate

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resist-like capabilities for EBL applications and beyond. The entire lithographic process takes place in a

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single instrument, and avoids exposing chemicals to the user and the need of cleanrooms. Unlike EBL

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that requires large samples with optically flat surfaces, we patterned on fragile membranes only 5-nm-

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thin, and 2 x 2 mm2 diamond samples. We created patterns on the nm to sub-mm scale, as well as three-

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dimensional structures by stacking layers of frozen organic molecules. Finally, using plasma etching, the

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organic ice resist (OIR) patterns are used to structure the underlying material, and thus enable

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nanodevice fabrication.

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Keywords

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Electron-beam lithography, condensed organic molecules, ice lithography, focused electron-beam induced

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deposition, 3D lithography, nanostructured diamond

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Text

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Electron-beam patterning at the nanometer scale enables research areas such as plasmonics and

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metamaterials1, optomechanical2 and nanofluidics3 systems, quantum computing4, advanced electronics

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and 2D material devices5. The electron beam lithography (EBL) process (Figure 1a) includes four steps: spin-

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coating resin-based resists on optically flat surfaces, resist baking, electron beam (e-beam) exposure, and

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development in liquid followed by sample drying. Crucial to the success of the lithography process, these

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steps require cleanroom environment for reduced particle contamination, and dedicated equipment and

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chemicals. Resist processing also restricts the choice of sample materials and geometries. Another e-beam

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based nanopatterning method is focused electron-beam induced deposition6 (FEBID, Figure 1b), in which a

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precursor gas is introduced into the EBL instrument through a nozzle in close proximity to the sample

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surface. The energetic e-beam dissociates the gas molecules adsorbed on the surface and deposits a solid

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at the exposure site. Most of the gas is unreacted and removed by pumps. FEBID’s ultimate resolution is

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better than EBL7-8, and the lack of spin-coating and development enables patterning on complex sample

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topographies and creating 3D nanostructures9. However, the simultaneous interaction between gas

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precursor, sample surface and the e-beam electrons to form the solid deposit results in processing times

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up to 5 orders of magnitude longer than EBL. In ice lithography10 (IL, Figure 1c), water vapour condenses

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into a solid layer of ice that coats the sample held at cryogenic temperatures. The e-beam removes the ice

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and uncovers the underlying substrate, and the resulting patterns are equivalent to those from a positive-

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tone EBL resist. To realize metal nanostructures, maintaining cryogenic conditions and vacuum, the sample

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is then subjected to a metallization and lift-off process where the patterned ice acts as mask. Since no resist

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application is required, IL allows nanopatterning on non-planar or fragile samples such as freely suspended

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carbon nanotubes11. Unfortunately, since most downstream nanofabrication instruments work at ambient

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conditions, the need for cryogenic in-situ processing limits IL applications.

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We discovered that frozen layers of simple organic molecules can be patterned by an electron beam for

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lithography applications. Most experiments were performed in a custom instrument consisting of a

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scanning electron microscope (SEM) modified to include a liquid nitrogen cooled cryostage, a gas injection

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system (GIS), an e-beam scan control system, and an airlock for sample exchange. Design, implementation

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and operation of the experimental setup are described in detail elsewhere12. In brief, to form the OIR film,

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we cool down the sample in the SEM vacuum, and we direct the organic gas through the GIS nozzle onto


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the cold sample surface, where it condenses to a uniform layer of ice. We irradiate the OIR thin-film with

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the e-beam, and then we transfer the sample to the airlock to heat it to room temperature while

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maintaining vacuum. In the e-beam exposed areas a solid product is formed, whereas the unexposed OIR

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sublimates within minutes, and it is removed by the vacuum system of the airlock (Figure 1d). The resulting

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patterns are stable at ambient conditions. As in IL and FEBID, this sublimation step completely replaces the

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solvent development needed in EBL. In strong contrast to IL, our e-beam patterned OIR can be used ex-situ

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for further characterization and processing, as the resulting patterns are equivalent to those made by a

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negative-tone EBL resist. Because the entire lithography happens in one single instrument, cleanroom

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environment and additional resist processing instrumentation are not required, thereby streamlining

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nanofabrication.

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Figure 1. OIR patterning compared to other e-beam based techniques. (a) EBL – The sample is coated with a resin layer that

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changes its solubility when exposed to the e-beam. The sample is then removed from the vacuum instrument and the pattern is

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developed with dedicated solvents. (b) FEBID – A precursor is introduced in the vacuum chamber through a nozzle, it is adsorbed

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onto the surface and dissociates due to the electron bombardment during exposure, depositing a non-volatile product. (c) IL –

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Water vapour is condensed onto a cryogenically cooled sample to form an ice thin-film, which is removed by the e-beam. The

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patterned ice is used as mask for in-situ processes before bringing the sample back to room temperature. (d) OIR – A vapour of a


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simple organic compound is first condensed onto the cooled sample to form a uniform layer. Its interaction with the e-beam locally

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modifies its chemical composition resulting in a non-volatile product. When the sample is heated to room temperature the

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unexposed OIR sublimates while the exposed patterns are stable, enabling ambient downstream processing.

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We carefully designed and tested OIRs condensation conditions and e-beam scan parameters. The OIR

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thickness is controlled by adjusting the GIS leak valve and the total deposition time. We selected and

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patterned OIRs using a representative subset of important organic chemistry building blocks: increasing

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hydrocarbon chain length, presence of aromatic rings, and of hydroxyl groups (Figure 2a; see also the

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comparative Table S1). 1-pentanol (C5H12O) had the lowest source vapour pressure in the GIS, and

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therefore it provides the lowest deposition rates enabling uniform ultrathin layers (7 nm). This is

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challenging with resin-based EBL resists. Anisole (methoxybenzene, C7H8O) and nonane (C9H20) ice resists

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readily formed 20 to 200-nm-thick layers, which made them ideal for lithography applications. The layers

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were condensed at substrate temperatures ranging from 120 K to 150 K without any observable difference

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in subsequent performances, indicating a robust process. We also patterned isopropanol (C3H8O) ice, but

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the results were hard to reproduce, possibly due to its lowest freezing point among the four compounds

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tested. The critical doses (D80, Figure 2b) of nonane ice for 5 and 20 keV e-beam exposures were reached at

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3 and 12 mC cm-2, respectively. The analysis of exposed thick layers revealed that OIR resist thickness does

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not affect the critical dose, as even at 5 keV the electrons interact with the ice layer throughout its entire

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thickness at each scan point. This enables the patterning of OIR layers ranging from a few to hundreds of

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nanometer thick using identical patterning time, and is in direct contrast to the gas-based FEBID method,

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which deposits matter in a monolayer-by-monolayer fashion, leading to a process time proportional to the

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volume of the deposited material. The OIR critical dose is already 100-fold lower than in IL, allowing

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nanoscale fabrication as well as sub-mm features (Figure 2c–d) which would be prohibitively time

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consuming using water ice. The OIR dose is significantly higher than PMMA (250 µC/cm2 at 19 keV), and

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other advanced EBL resists (10 – 100 µC/cm2)13, and it is difficult to compare to FEBID doses, as these

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depend on precursor gas flow rate, nozzle and sample geometry, and writing direction14 as well as desired


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thickness of the deposits. Nevertheless, a benchmark figure for the FEBID dose required to deposit a single

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atomic monolayer from an organometallic precursor translates to 3 mC cm-2 14, which is equivalent to the

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dose we used to pattern the 600-nm-thick OIR layer in Figure 2b. Hence, to deposit layers above 100-nm-

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thick, as those used for etching mask applications, OIR is up to 3 orders of magnitude more efficient than

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FEBID.

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To investigate the resolution of OIR, we patterned and compared single line scans on OIR thin-films (Figure

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2e–g) and on the negative-tone EBL resist AR-N 7520. We obtained down to 60-nm-wide OIR lines, and 80-

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nm-wide EBL resist lines. The unexpectedly large EBL resist patterns indicate that feature size is currently

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limited by our custom instrument.

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Figure 2. OIR patterns. (a) Atomic force microscopy (AFM) images of 20 x 12 µm2 rectangles patterned in OIR thin-films and

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representative scan-lines across the rectangle surface. Average pattern thickness is in parenthesis. (b) Contrast curves for nonane

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ice at 5 and 20 keV. The critical dose at 5 keV is unchanged even with a 20-fold time increase in thickness. (c, d) Large area map of

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Denmark made by exposed nonane ice (dark blue areas). The total exposure time was 75 min. The optical image (c) covers an area

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of 0.4 x 0.5 mm2. The micrometric DTU logo (9 x 6 µm2) patterned in the Copenhagen region is visible in the SEM inset (d). (e) SEM


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image of 100-nm-wide lines patterned in nonane ice. (f) 60-nm-wide line patterned in anisole ice. (g) Tilted SEM view of patterned

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OIR lines reaching the edge of a 1 x 1 cm2 chip. The OIR layer is uniform on the entire sample surface, up to the very edge. Scale bar

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is 200 nm.

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To further probe the resolution performances of OIRs, we used a transmission electron microscope (TEM)

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fitted with a gas cell and a cryostage to condense OIR layers on electron-transparent membranes and

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perform analogous lithography experiments operating the TEM at 80 keV. First, we used our custom

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instrument to test whether OIR is compatible with the delicate 3-mm-wide TEM substrates consisting of 5-

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nm and 50-nm-thick silicon nitride membranes with holes. We deposited and patterned OIR layers on the

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membranes (Figure 3a; see also Figure S1) without any modification to the setup, while spin-coating EBL

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resist on such samples would have required custom holders and careful handling or, for more irregular

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surfaces, alternative strategies such as thermal evaporation of the resist15, which however relies on

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additional high temperature processes and still requires development in liquid solvents. Liquid

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development is undesired because the interfacial forces involved in the drying process may damage fragile

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nanostructures. To recreate the experimental conditions of OIR lithography in the TEM, we introduced

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nonane vapour through the GIS connected to the gas cell, condensing OIR thin-films on cold 5-nm-thick

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silicon nitride membranes. We focused the TEM e-beam to scan and expose patterns in the form of dots

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arrays. After exposure, we heated the cryostage in-situ and immediately observed the resulting structures

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(Figure 3b). The significantly improved feature size (down to 10 nm, Figure 3c) confirms that our SEM

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results are far from the ultimate resolution of this technique, and suggests that by improving the optics and

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stability of our custom instrument we might achieve with OIRs critical dimensions in line with EBL.


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Figure 3. OIR patterning in an environmental TEM. (a) TEM bright-field image of a 20x20 µm square and lines previously patterned

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in our custom instrument on a 20-nm-thick “holey” Si3N4 membrane. (b, c) TEM dark-field (b) and bright-field (c) images of exposed

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nonane OIR dots patterned on 5-nm-thick Si3N4 membranes. In this case, OIR condensation, exposure, sublimation and imaging

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were all performed inside the TEM using its 80 keV beam. Scale bar in (b) is 500 nm, the highlighted dots in (c) are 18 and 10 nm in

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diameter.

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Another major advantage of OIR is 3D patterning capabilities, which we demonstrate with multi-layered

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structures realized in our custom instrument using an iterative condense-exposure process (Figure 4a; see

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also Supporting Movie 1 for a 3D animation). All three OIR layers are uniform within sub-nanometer surface

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roughness and unaffected by the stacking, which suggests that many layers could be added for more

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complicated and higher aspect ratio structures. We then modified this 3D lithography process using

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condense-expose-sublimate cycles. Using the first 3D process, the top layers present the cumulative

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thickness as in an additive manufacturing process. In the modified process, any additional top layer follows

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the profile of the previously exposed features underneath (Figure 4b) and allows multiple patterning, which

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is an important technique used to increase pattern density16. For these configurations, OIR processing is

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more straightforward than EBL, because all steps can be performed in-situ while EBL requires multiple

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resist spin, bake and development steps. Furthermore, EBL resists such as PMMA cannot be stacked in the

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first place, because the solvents for spin-coating damage the underlying features. Additive 3D routines in

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low temperature FEBID were introduced by Bresin et al.17, who reported on optimizing growth

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performances for direct writing of platinum structures up to several micrometers thick. The deposits are


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platinum clusters embedded in an amorphous carbon matrix, and the method appears powerful for direct

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writing of metallic nanostructures. However, platinum deposits are not suitable for nanoscale lithography

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applications since, unlike organic resists, platinum cannot be removed by chemical or physical processing

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without relying on aggressive chemistries.

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Figure 4. Three-dimensional multi-layered structures. (a) AFM 3D image and its central scan line for a multi-layered structure

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obtained by patterning 3 layers of anisole ice. (b) OIR-enabled approaches to multilayer patterning, and AFM images of the results.

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In the AFM images, L1 originates from exposing the first ice layer, and L2 from the second. Scale bar is 1 µm.

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To create useful devices, lithographic patterns must be transferred into an underlying functional material.

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The lithography patterns act as masks during pattern transfer, and they must be chemically stable. Indeed,

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OIR patterns were stable when placed in different organic solvents, and mild acids and bases. We used OIR

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patterns as protective etch masks in reactive ion etching to create silicon nanostructures (Figure 5a-b, see


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Supporting Information for details of the etch process). A good etch mask must not be consumed during

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etching of the underlying material, with their relative etch rate defined as the selectivity. For nonane ice

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patterns, selectivity over silicon was 1:6, which is the same value obtained with AR-N 7520. Using oxygen

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plasma, we can remove the patterned OIR without leaving any residues or harming the underlying silicon.

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This ensures a pristine surface for further processing or final applications. The good chemical stability, high

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selectivity, and the fact that they can be removed by oxygen plasma, suggest that these features might be

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amorphous hydrogenated carbonaceous solids akin to FEBID deposits18-19. EDX and EELS analysis of the

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exposed OIR on the TEM grids showed carbon and oxygen (Figure S2), but both methods are not directly

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sensitive to the expected hydrogen content.

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One main advantage of OIR is that we can handle small samples such as the TEM membrane samples used

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in our TEM experiments. As a further demonstration, we patterned OIR onto diamond chips as small as 2 x

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2 mm2 to be used as etch mask (Figure 5c-e). Diamond substrates and nanopillars are e.g. used for

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nitrogen-vacancy centres for quantum technology applications20. In contrast to EBL resist coating, uniform

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OIR layers of well-controlled thickness can be deposited on the entire sample surface, and the one-step

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lithography reduces sample manipulation to a minimum.


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Figure 5. Fabrication of nanowires and nanopillars by plasma etching. (a) AFM profiles evolution of OIR lines on a silicon substrate

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at three different steps in the etch process: as patterned, after silicon etch, and after removal of the residual OIR. The etch

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selectivity between the patterned OIR and silicon is 1:6. (b) SEM view of 400-nm-tall silicon fins made with OIR and reactive ion

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etching. Scale bar is 500 nm. (c) Tweezers holding a 2 x 2 mm diamond chip used for fabricating diamond quantum devices. (d)

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SEM view of an array (pitch 10 µm) of diamond nanopillars fabricated using patterned OIR as the etch mask. (e) Close-up of

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diamond pillars. Scale bar is 1 µm.

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In conclusion, OIR offers unique advantages to nanofabrication beyond the state-of-the-art. These ice

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layers of simple organic molecules can be used as negative-tone resists in e-beam based patterning without

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requiring spin-coating or developing in liquid solutions. Both larger sub-mm- and nanostructures can be

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created by condensing these molecules on a cooled substrate and scanning the e-beam according to the

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desired pattern. The unexposed organic ice sublimates away, leaving no residues, while the exposed areas

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are carbonaceous structures stable at ambient conditions, which, if needed, can be removed in a gentle

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oxygen plasma step. After the lithography process, the initial substrate remains pristine and undamaged.

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Similar to additive manufacturing technologies, we also made 3D structures by stacking multiple layers.

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While we used homogeneous OIR stacks in these experiments, we believe there is unexplored potential in

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combining chemistries and advanced 3D structures consisting of heterogeneous multilayer stacks. Still to be

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discovered are e.g.; organic compounds providing desirable electrical properties for the resulting features;

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additional interlayer dynamics (e.g. chemical reactions, sacrificial layers) which could further increase the

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complexity of 3D structures and the range of potential applications.

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We have only started to explore the potential of OIR for scientific research. To identify the most suitable

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molecules and processes for future applications and challenges, we are now focusing our research efforts

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on achieving in-depth scientific understanding of electron-OIR interactions.

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Supporting Information

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Supporting information is available in the online version of this paper:

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Experimental details regarding sample preparation, experimental setup for cryogenic SEM and TEM lithography and

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imaging, chemical stability and pattern transfer, diamond nanofabrication and further characterization; comparison of

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different precursors; TEM analysis of OIR patterns after fabrication.

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Animation video showing an example of iterative 3D lithography process, used to create the pattern in Figure 4a.

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Author information

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Corresponding author

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Email: [email protected]

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Author contributions

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Experimental work on OIR patterning and characterization was carried by W. T., as conceived and discussed together with A. H. and

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M. B.; A. E. and P. L. performed in-situ TEM experiments and other TEM characterization. H. T. L. performed the work on OIR

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chemical stability, dry etching and characterization of the etched features. W. T. drafted the manuscript; all authors contributed to

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revisions and comments and discussed the results.

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Notes

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The authors declare no competing financial interests.


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Acknowledgments

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The authors would like to acknowledge J. E. Jørgensen, M. Nimb, T. Feld and S. M. B. Petersen for providing custom parts in the

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experimental setup. The authors thank for their contribution, assistance and valuable inputs A. Gregersen (for the electronics

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setup), R. Cork (vacuum setup), J. Michael-Lindhard (SEM instrument), K. H. Rasmussen (diamond etching). We are also grateful to

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D. Branton for helpful comments. We also acknowledge financial support from the VILLUM foundation.

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References

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