Electron Beam Lithography

Direct write electron-beam (e-beam) lithography, which has the maskless patterning capability and the quick turnaround for new device designs and design changes, has been applied to making the engineering samples for the development of the System on Chip products (SoC). Using the e-beam lithography to the multilevel interconnect metal was known to be evaluate in view of cost and throughput. In the case of the high-energy e-beam lithography, however, the backscattered electron from the metal caused a significant proximity effect. Authors evaluated the e-beam proximity effect using the accelerating voltage 50keV on some multi-level interconnect metal structures which consist in tungsten wiring, or Cu wiring. It was found that the backscattering range and the ratio of the backscattering energy to the incident energy depend on the thickness of metal, but also on the distance from the resist to the metal. Therefore authors propose a new method of evaluating e-beam lithography property, concept of "EB-tree". That indicates the wafer backscatter property that has heavy metal wiring using e-beam lithography. EB-tree shows the relations of wafer backscatter range and heavy metal thickness, ratio of the backscattering energy and heavy metal thickness. EB-tree could show wafer property cause of lower levels layout, understructure metal wiring, that must be taken into account when e-beam lithography.

We report the nanofabrication and characterization of x-ray transmission gratings with a high aspect ratio and a feature size of down to 65 nm. Two nanofabrication methods, the combination of electron beam and optical lithography and the combination of electron beam, x-ray, and optical lithography, are presented in detail. In the former approach, the proximity effect of electron beam lithography based on a thin membrane of low-z material was investigated, and the x-ray transmission gratings with a line density of up to 6666 lines/mm were demonstrated. In the latter approach, which is suitable for low volume production, we investigated the x-ray mask pattern correction during the electron beam lithography process and the diffraction effect between the mask and wafer during the x-ray lithography process, and we demonstrated the precise control ability of line width and vertical side-wall profile. A large number of x-ray transmission gratings with a line density of 5000 lines/mm and Au absorber thickness of up to 580 nm were fabricated. The optical characterization results of the fabricated x-ray transmission gratings were given, suggesting that these two reliable approaches also promote the development of x-ray diffractive optical elements.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Pattern transfer by deep anisotropic etch is a well-established technique for fabrication of nanoscale devices and structures. For this technique to be effective, the resist material plays a key role and must have a high resolution, reasonable sensitivity, and high etch selectivity against the conventional silicon substrate or underlayer film. In this work, the lithographic performance of two high etch resistance materials was evaluated: ZEP520A (Nippon Zeon Co.) and mr-PosEBR (micro resist technology GmbH). Both materials are positive tone, polymer-based, and nonchemically amplified resists. Two exposure techniques were used: electron beam lithography (EBL) and extreme ultraviolet (EUV) lithography. These resists were originally designed for EBL patterning, where high quality patterning at sub-100 nm resolution was previously demonstrated. In the scope of this work, the authors also aim to validate their extendibility to EUV for high resolution and large area patterning. For this purpose, the same EBL process conditions were employed at EUV. The figures of merit, i.e., dose to clear, dose to size, and resolution, were obtained, and these results are discussed systematically. It was found that both materials are very fast at EUV (dose to clear lower than 12 mJ/cm2) and are capable of resolving dense lines/space arrays with a resolution of 25 nm half-pitch. The quality of patterns was also very good, and the sidewall roughness was below 6 nm. Interestingly, the general-purpose process used for EBL can be extended straightforwardly to EUV lithography with comparably high quality and yield. Our findings open new possibilities for lithographers who wish to devise novel fabrication schemes exploiting EUV for fabrication of nanostructures by deep etch pattern transfer.

Background: Fast advancing of microwave/terahertz communication constantly demands ultrashort T-shaped gates by innovative nanofabrication techniques. The emerging RE650 resist provides a promising alternative to existing resists owing to its outstanding performance in electron-beam (e-beam) lithography. Aim: To report the exploration of the minimum foot-width achievable by e-beam lithography on RE650/UV5 bilayer resists through both numerical simulation and experimental tests. Approach: Using Monte Carlo method, the spatial charge distributions by e-beam exposure in resists were calculated, followed by modeling the developing process using LAB software to obtain the lithography profiles in resists. By this way, the foot-width was compared for the RE650 thickness varying from 50 to 80 nm with various UV5 thickness fixed. Results: As narrow as 15-nm foot-width has been achieved, but 30-nm foot-width can be routinely replicated in a stable exposure latitude, indicating that the developed process for ultra short T-shaped gates is reliable. Conclusions: The bilayer of RE650/UV5 is capable of replicating 15-30-nm T-shape gates as the minimum width by e-beam lithography, which should give us high prospect for mass production of THz devices.

One of the major limiting factors in electron beam (e-beam) lithography is the undesired development of circuit area caused by electron scattering (the proximity effect). Since the proximity effect imposes a severe limitation on the ultimate resolution attainable with electron beam lithography, it is important to examine ways in which the proximity effect can be reduced. In order to develop a fast and efficient proximity effect correction scheme it is necessary to have a fast and accurate technique for finding the exposure (energy) deposited at any given point in the circuit pattern. In this paper we present the hierarchical digital signal processing (DSP) model utilized by PYRAMID (our proximity effect correction scheme) to estimate these exposures. Our approach employs a two part hierarchical estimation technique which is several orders of magnitude faster than a direct (spatial domain or FFT-based) convolution yet is able to predict the result of an exposure with a high degree of accuracy. A detailed description of our method of exposure estimation is followed by a thorough analysts of its performance through simulation and experimental results.

Organic contaminants adsorbed on the surface of a silicon stencil mask in a high-vacuum chamber in an electron-beam exposure lithography system have been analyzed and identified using thermal-desorption gas chromatography/mass spectrometry (TD-GC/MS), X-ray photoelectron spectrometry (XPS), and electron energy loss spectroscopy (EELS). The major organic compound adsorbed onto the surface of the silicon stencil mask in the high vacuum chamber has been found to be di-2-ethylhexyl phthalate (DOP). The amount of DOP adsorbed onto the silicon surface in high vacuum is much larger than that adsorbed in ambient air. A relatively high molar ratio and long mean free path of organic molecules with a low vapor pressure, such as DOP, in high vacuum is considered to enable them to be easily adsorbed onto the silicon surface in high vacuum. On the other hand, organic volatiles with higher vapor pressure are not adsorbed on the silicon surface in high vacuum. It has been found by XPS and EELS that the organic contaminants adsorbed on the surface of the silicon stencil mask are transformed into graphite-like carbon by electron beam irradiation during the exposure process.

DNA oligonucleotides of different sequences were patterned at the nanoscale. Areas of positive charge were generated by exposure of insulating substrates, spin-on hydrogen silsesquioxane or vapor-deposited SiO(2) on Si, with ionizing radiation sources used in electron beam and extreme ultraviolet lithography. Au nanoparticles (NPs) with a diameter of 15 nm, carrying covalently bound negatively charged single-stranded DNA oligonucleotides, were site specifically immobilized directly on the exposed regions and presented oligonucleotides for subsequent hybridization. Repeated exposure and deposition of NPs allowed for patterning multiple DNA sequences. Patterns with dimensions as small as 15 nm were fabricated using electron beam lithography. The use of DNA-functionalized NPs rather than just DNA facilitates metrology in scanning electron microscopy and improves the hybridization efficiency of the oligonucleotides on the surface.

The high-volume production of semiconductor devices with sub-10 nm critical dimensions is challenging. We have investigated the feasibility of the fabrication of line-and-space patterns with a 7 nm quarter-pitch (7 nm space width and 21 nm line width) by electron beam (EB) lithography. In this study, the optimum beam size for the fabrication of line-and-space patterns with a 7 nm quarter-pitch was investigated from the viewpoint of the trade-off relationship between line edge roughness (LER) and sensitivity. When the peak charge was constant, the optimum beam size depended on the required sensitivity. When the total charge was constant, the beam size was required to be less than 1.6 nm for minimizing LER.

The development of a ferroelectric negative tone sol–gel resist for Ultraviolet (UV) and Electron Beam (EB) lithography is presented. A new system based on Lead Zirconate Titanate (PZT, with formula PbZr0.52Ti0.48O3) was synthesized by sol–gel method. The lithographic performances were investigated and several structures spanning from the micron range down to less than 50 nm have been achieved by UV and EB lithography. The system interaction with UV light and Electron beam was thoroughly characterized by FT-IT spectroscopy. The exposed PZT was annealed at high temperatures in order to study the crystalline phase evolution, the optical constants values and stability of patterned structures. After exposure and annealing, the refractive index of the material can vary from 1.68 up to 2.33 (@400 nm), while the ferroelectric behaviour seems to be maintained after high temperature annealing. These results suggest a possible application of PZT resist not only as ferroelectric but also as nanopatternable high refractive index material. Moreover, direct nanopatterning by means of Focused Ion Beam (FIB) lithography was verified and the potentiality for the preparation of high aspect ratio hollow nanostructures will be presented.

Electron beam (EB) lithography using polymethylmethacrylate (PMMA) andoxygen gas reactive ion etching (RIE) were used to fabricate fine patterns in adiamond mould. To prevent charge-up during EB lithography, thin conductivepolymer was spin-coated over the PMMA resist, yielding dented line patterns 2μm wide and 270 nm deep. The diamond mould was pressed into PMMAon a silicon substrate heated to 130, 150 and 170ºC at 43.6, 65.4 and87.2 MPa. All transferred PMMA convex line patterns were 2 μ mwide. Imprinted pattern depth increased with rising temperature andpressure. PMMA patterns on diamond were transferred by the diamondmould at 150ºC and 65.4 MPa, yielding convex line patterns 2 μm wide and 200 nm high. Direct aluminium and copper patterns wereobtained using the diamond mould at room temperature and 130.8 MPa. Thediamond mould is thus useful for replicating patterns on PMMA and metals.

Attempts were made to form regular arrays of size- and position-controlled Pt-dots on GaAs and InP by combining an in situ electrochemical process with the electron beam (EB) lithography. This utilizes the precipitation of Pt nano-particles at the initial stage of electrodeposition. First, electrochemical conditions were optimized in the mode of self-assembled dot array formation on unpatterned substrates. Minimum in-plane dot diameters of 22 nm and 26 nm on GaAs and InP, respectively, were obtained under the optimal pulsed mode. Then, Pt dots were selectively formed on patterned substrates with open circular windows formed by EB lithography, thereby realizing dot-position control. The Pt dot was found to have been deposited at the center of each open window, and the in-plane diameter of the dot could be controlled by the number, width and period of the pulse-waveform applied to substrates. A minimum diameter of 20 nm was realized in windows with a diameter of 100 nm, using a single pulse. Current-voltage (I-V) measurements using an atomic force microscopy (AFM) system with a conductive probe indicated that each Pt dot/n-GaAs contact possessed a high Schottky barrier height of about 1 eV.

Electron beam lithography (EBL) is widely used for patterning of sub-micron and nano-scale patterns. Patterns in the order of tens of nano meters have been successfully realized using EBL. There are increasing needs in high aspect ratio structures in sub-micron and nano scales for microelectronics and other applications. Traditionally, high aspect ratio structures in sub-micron and nano scales have been realized by precision lithography techniques and subsequent dry etch techniques. In this work, we present commercially available SU-8 as a potential resist that can be used for direct resist patterning of high aspect ratio structures in sub-micron and nano scales. Such resist pattern can be used as a polymeric mold to create high aspect ratio metallic sub-micron and nano scales structures using electroplating technology. Compared to the most commonly used EBL resist, PMMA (poly methylmethacrylate), SU-8 requires a factor of 100~150 less exposure doses for equal thickness. It results in a significant reduction of EBL processing time. In this paper, characterization results on the patterning of up to 4:1 aspect ratio SU-8 structures with minimum feature size of 500 nm is reported. In addition, preliminary results on high aspect ratio metallic sub-micron structures using electroplating technology are also reported.

Low-energy e-beam proximity lithography (LEEPL) is proposed as the simplest integrated circuit lithography for minimum feature sizes ≤0.1 µm. This new e-beam lithography is similar to 1× X-ray proximity lithography except that the X-ray beam is replaced with a beam of low-energy electrons of 2 kV. This low e-beam energy permits the use of single-crystal 0.5-µm-thick silicon stencil masks without an absorbing metal layer of high atomic number. This membrane mask is thick enough for good heat conduction and thin enough for feature sizes ≤0.1 µm. Mask distortion caused by fabrication can be corrected by a fine-tuning deflector. Therefore, a mask with a residual distortion of more than 100 nm is acceptable. This eliminates the main difficulty of X-ray proximity lithography. The proposed system is not affected by a space-charge effect in the electron optics column, and a proximity effect with respect to both wafer and mask writings, and it is fundamentally low-power lithography which needs no special cooling system. The analysis shows that the e-beam column can be made entirely of electrostatic components to achieve sufficient resolution. For an appropriate resist process for this low-energy e-beam, we propose a bilayer process such as the chemical amplification of resist lines (CARL) process which consists of a chemically amplified thin deep ultraviolet (DUV) photoresist and a thick planarizing layer as a starting point. We estimated a throughput of about 40 12 inch wafers per hour and a resolution of a significantly less than 50 nm.

Research at the US Naval Research Laboratory (NRL) in the lithography, patterning, and fabrication of structures of nanometer scale dimensions is reviewed. Electron-beam (e-beam) lithography at high (50 keV) and low (5 eV) energies on resist systems developed at NRL is described. The low energy lithography takes advantage of the spatially confined e-beam available in a scanning tunneling microscope type probe. This instrument allows an in situ exposure and characterization of an e-beam sensitive material. A novel approach for implementing dose correction for proximity effects in e-beam lithography is presented using an error measure based on the physical realities of lithography. The compositional disordering of GaAs-Ga/sub x/Al/sub 1-x/As heterostructures as a technique for pattern replication is described. Introducing silicon into the heterostructure (by implantation or diffusion) enhances the aluminum and gallium interdiffusion which can be used for patterning. A complete bibliography of recently published results is included.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>