K. W. Rhee

Imaging layers for 50 kV electron beam lithography: Selective displacement of noncovalently bound amine ligands from a siloxane host film

We report the development of an imaging layer technology for 50 kV electron-beam lithography based upon the displacement of noncovalently bound amine ligands from a siloxane host film. The patterned films were used as templates for the selective deposition of an electroless nickel film resulting in a positive tone imaging mechanism. The deposited nickel was sufficiently robust to function as an etch mask for pattern transfer by reactive ion etching. Metallized and etched patterns with linewidths to approximately 40 nm are demonstrated using an exposure dose of 500 μC/cm2.

Sub-0.1 μ electron-beam lithography for nanostructure development

This article discusses those factors that lead to resolvability in e-beam lithography. The primary tool of study is the simplex method of linear optimization theory. Resist exposure characteristics can effectively be accounted for using a “minimum contrast” approach. The simplex exposure technique does not require exposure in the normally unexposed field. Feature linearity comparison experiments and boundary placement studies show measurable improvement with simplex when compared to dose scaling or feature boundary resizing. While computational tractability is a significant issue, the technique will rapidly assess whether small groups of features are resolvable. In addition, simplex points to general rules for applying dose modulation to different classes of features.

Modeling of electron elastic and inelastic scattering

The role of the form of the elastic and inelastic cross section in Monte Carlo simulations of electron–solid scattering has been studied to understand the processes whereby energy is deposited by electrons as they traverse thin films. Specifically we are interested in these phenomena as they relate to proximity effects in electron‐beam lithography and the detection of electrons by a Schottky diode with a patterned absorber overlayer. Lithographic point and line spread functions have been measured in three resist materials. We show that the inclusion of discrete inelastic scattering events whereby fast secondaries are generated is essential for matching simulation and experiment. The secondaries are crucial in determining the shape of the spread functions in the 0.1–1 μm regime and must be included to model proximity effects. Further, the fitting of line spread function simulations to experiment allows the accurate prediction of dot spread functions and applied dose thresholds as well as three dimensional ...

An error measure for dose correction in e‐beam nanolithography

In this paper we address the problem of dose correction in ultrahigh resolution electron beam lithography. Here, the emphasis is on providing precise resist development profiles. This contrasts with the case of standard near‐micron e‐beam lithography in which computational efficiency and an ability to handle large data files are the major goals. The approach employed here is one of non‐linear optimization. Rather than using the conventional quadratic cost function for dose optimization, the specifics of the resist development curve (i.e., saturation behavior, minimum critical dose, and gamma) are incorporated as inequalities. Results (both experimental and theoretical) are given for half‐micron lines. Preliminary results on subhalf‐micron lines are also given.

Simultaneous measurement of gap and superposition in a precision aligner for x‐ray nanolithography

Previously we described an x‐ray mask alignment system, capable of nanometer‐level superposition precision, whose alignment signal did not appear to be adversely affected by overlayers of resist, polysilicon, or metal [E. E. Moon, P. N. Everett, and H. I. Smith, J. Vac. Sci. Technol. B 13, 2648 (1995)]. The system, called interferometric broad‐band imaging (IBBI), employs grating and grid type alignment marks on mask and substrate, respectively. These are viewed at an inclined angle, through the mask, using f/10 optics with a working distance of 110 mm. The inclined angle and long working distance avoid interruption of the x‐ray beam. Using a charge‐coupled‐device camera, misalignment is measured from two identical sets of interference fringes (∼50 μm period) that move in opposite directions as the mask is moved relative to the substrate. Alignment corresponds to matching the spatial phases of the two sets of fringes. Here we demonstrate that the same alignment optics and grating type alignment mark on th...

Comparative mobility degradation in modulation‐doped GaAs devices after e‐beam and x‐ray irradiation

We report on measured Hall mobility versus temperature for high‐quality modulation‐doped AlGaAs/GaAs samples after exposure by electrons and x rays at doses and energies typically used in lithography. We find that bare samples exposed by 50 keV electrons suffered significant mobility degradation over the temperature range of 4.2–300 K (as much as a factor of 30). X‐ray‐exposed samples did not show any mobility degradation. Two‐dimensional electron densities were not dramatically affected by either exposure technique, although e‐beam exposed samples did show a slight decrease in carrier density. Our results are consistent with previous reports of mobility degradation in some e‐beam evaporators.

Electron-beam fabrication of nonplanar templates for contact printing

Templates for contact printing applications have been fabricated on convex and concave surfaces using electron-beam lithography. Curved quartz lens blanks, coated with a chrome layer to suppress charging, were spincoated with a layer of polymethylmethacrylate (PMMA). Features were patterned into the PMMA followed by development, pattern transfer into the chrome by wet etch, and reactive-ion etching (RIE) of the chrome masked quartz. Features to 0.3 μm have been patterned and transferred ∼600 nm into the underlying quartz substrate by RIE, with less than a 10% etch depth variation across the sample. The patterned substrate was then used as a template to cast polymer stamps from poly(dimethylsiloxane). This approach provides a convenient method of fabricating curved templates for contact printing.

Reactive ion etching of high‐aspect‐ratio 100 nm linewidth features in tungsten

The subtractive patterning of fine‐linewidth (sub‐250‐nm) tungsten absorbers for x‐ray masks requires the ability to etch features with high aspect ratios and vertical sidewalls. In this letter, a reactive‐ion‐etching process which meets these requirements is described. The etch gases used are SF6 and H2. It is further shown that an intermittent etch process, whereby the sample is vented to atmosphere in between etches results in an etch profile with vertical tungsten sidewalls. This interrupted etching is compared with the results from continuous etching. The straighter sidewalls in the case of the interrupted etch suggests a passivation of the sidewalls which occurs during the venting process. Results are presented to show etching of 100 nm gratings and 200 nm dot arrays. An in situ endpoint detection method developed for the reactive‐ion‐etching system is presented.

Fabrication of parallel quasi‐one‐dimensional wires using a novel conformable x‐ray mask technology

We report on the fabrication of quasi‐one‐dimensional wires on modulation‐doped GaAs/AlGaAs using a novel conformable x‐ray mask technology which allows us to expose arbitrary sized samples, including samples much smaller than the membrane area, using our laboratory’s standard 31 mm‐diam silicon‐nitride x‐ray mask. After optical alignment, the sample and mask are brought into contact electrically, and then loaded into a specially designed cartridge which allows a vacuum to be pulled between mask and substrate. The vacuum causes the x‐ray mask to conform around the sample. We find that a vacuum hold down is necessary to allow easy separation of the sample from the mask with minimal risk to both.

Conductance quantization in a GaAs electron waveguide device fabricated by x-ray lithography

We report on the fabrication of AlGaAs/GaAs split‐gate electron waveguide devices of lengths between 0.1 and 2 μm using x‐ray lithography, and the measurements of these devices at liquid‐helium temperatures and up to 15 K. An x‐ray mask (parent mask) was fabricated using e‐beam lithography and replicated using proximity x‐ray lithography (λ=1.32 nm) to generate a replica (daughter) mask. The daughter mask was then aligned to patterns on a high‐mobility AlGaAs/GaAs sample and x ray exposed using a conformable mask fixture. The conductance of the electron waveguides was measured as a function of the split‐gate bias. Sharp 2e2/h conductance steps were observed in devices up to 0.75 μm long at T=2 K. The features in the conductance remain visible up to 15 K.