Highly organized monolayers formed from the self-assembly of octadecyl derivatives on oxide-covered Si and Ti substrates have been exposed to electron beam impact under typical conditions used in lithographic patterning. A combination of X-ray photoelectron spectroscopy, ellipsometry, infrared spectroscopy, and liquid drop contact angle measurements show that the major effect of irradiation is the loss of H, Via cleavage of C-H bonds, to form a carbonaceous residue with a surface containing oxygenated functional groups.
Dots demonstrating critical resist dimensions of approximately 5 to 6 nm were formed in an octadecylsiloxane monolayer on silicon by electron beam exposure using a digital scanning electron microscope at 20 keV beam energy. The patterned dots were observed by imaging with an atomic force microscope (AFM). The electron beam size was measured to confirm that it is not the limiting factor in the exposure resolution. The limit that prevents the observation of smaller structures is either the small contrast in the AFM imaging for smaller dots or an intrinsic material limit caused by the secondary electron range.
We present results on electron beam exposure of a self-assembled monolayer film as a self-developing positive resist on GaAs. A 1.5 nm thick monolayer of n-octadecanethiol (C18H37SH) deposited on a GaAs (100) substrate showed a electron beam sensitivity of about 100 μC/cm2. The monolayer resist was used as a mask for chemical etching of the GaAs. Patterns in GaAs have been created with widths approximately equal to the exposing electron beam width of 50 nm.
Self-assembled monolayers have been modified with focused electron beams of energy 1–50 keV and scanning tunneling microscopy (STM) based lithography with energies of ∼10 eV. Modifications ∼15 nm in size have been formed by STM and ∼25 nm in size by 50 keV beams. The fact that these materials work as self-developing electron beam resists is demonstrated by both atomic force microscopy imaging and pattern transfer using conventional wet etchants. Patterns have been transferred to silicon substrates to a depth of ≳120 nm with a multistep wet etching process. The mechanism of electron beam modification has also been explored to better design future monolayer processes.
We have demonstrated that low energy electron beams (≤10 eV) from a scanning tunneling microscope (STM) can be used to modify a surface of self-assembled monolayers of octadecanethiol [ODT, CH3(CH2)17SH] on gold and GaAs. STM modification in air was used to produce grating patterns up to 140 μm in total size. Line sizes as small as 15 nm were produced in ODT on GaAs at a bias of 10 V, and slightly larger sizes were produced on the gold substrate. Biases of greater than 4 V are necessary for the fabrication of these raised lines, as observed with an atomic force microscope (AFM). The patterns in ODT on gold were successfully transferred into the gold layer with a wet chemical etch demonstrating that the monolayer performs as a positive electron beam resist.
Electron-beam lithography with aromatic self-assembled monolayers on silicon surfacesIt was demonstrated that self~assembled monolayers of n-octadecanethiol [ODT; CH3(CH2)I7SH] on GaAs and n-octadecyltrichlorosilane [OTS; CH 3 (CH 2 ) 17SiC13] on Si0 2 act as self-developing positive electron beam resists with electron-beam sensitivities of ~ 100-200 p,C/cm 2 • For the OTS monolayer on a silicon native oxide, atomic force microscopy (AFM) images of the exposed layer before etching demonstrate the removal of all or part of the layer upon electron~beam exposure. Features as small as 25 nm were resolvable in a 50 nm period grating. A resist contrast curve for OTS was obtained from AFM depth measurements as a function of dose. An ammonium hydroxide water etch was used to transfer patterns into the GaAs to a depth of at least 30 nm and buffered HF was used for pattern transfer into the Si0 2 to a depth of at least 50 nm.
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High resolution electron beam lithography using ZEP-520 and KRS resists at low voltageSingle crystal and polycrystalline silicon films have been patterned and etched with a novel high-selectivity process using self-assembled monolayer resists of octadecylsiloxanes ͑ODS͒. The highest resolution patterning of sub-10 nm features has been demonstrated by scanning force microscopy imaging of ODS layers patterned with a focused electron beam. An all-dry UV/ozone developer has been used to remove residual carbon from the electron beam exposed regions to improve etch selectivity. The positive tone pattern transfer process consisted of a short buffered hydrofluoric acid wet etch to remove the silicon native oxide followed by a high-selectivity, low ion energy etch using Cl 2 and BCl 3 in an electron cyclotron resonance reactive ion etch. Features have been etched up to 90 nm deep into Si͑100͒ wafers and minimum feature sizes obtained are ϳ25 nm. Poly-Si films on SiO 2 insulator layers have been similarly patterned and have been used in a combined process with photolithographic definition of microbridges to form narrow conducting channels in the poly-Si.
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