A. N. Broers

Electron beam lithography-Resolution limits

Electron beam lithography is generally accepted to have the highest practical resolution capability. In this paper the state of the art in terms of resolution is reviewed. This covers conventional resists such as PMMA, contamination resist, inorganic resists and damage processes. Some insights into the resolution limiting factors are given although the precise limitations still remain unclear. Consideration is also given as to the best measure of resolution and it is suggested that the minimum line spacing is a more appropriate and less subjective measure than minimum achievable feature size.

Direct nanometer scale patterning of SiO2 with electron beam irradiation through a sacrificial layer

Nanometer scale patterns have been fabricated in SiO2 by electron beam exposure through a sacrificial layer. Although the process of patterning SiO2 with direct electron beam irradiation was discovered over two decades ago, the smallest feature size previously achieved was 0.6 μm because finely focused electron beams form a contamination layer on the substrate blocking the subsequent development of the oxide with HF wet etches. Exposing through a sacrificial layer, the contamination is readily removed with the stripping of the sacrificial layer. Using high energy electrons (300 keV) to minimize forward scattering of the beam, arrays of lines with a period down to 21 nm have been fabricated in the SiO2.

Negative PMMA as a high-resolution resist - the limits and possibilities

Poly(methylmethacrylate) (PMMA) which is commonly used as a positive resist can also be used in a negative manner with exposure at higher dose levels. In this paper we investigate the full potential of this resist for high-resolution pattern definition. We show that although the point spread exposure distribution is similar to that for positive PMMA, features of the order of 10 nm are easily achieved. These resist structures can be transferred into the underlying materials using plasma etch techniques to a similar degree of resolution. The density of resist features is also greater than for positive PMMA with features of nm in width being possible on a 30 nm pitch. The instability of the resist structures at extreme linewidths has been identified as a potential problem in utilizing the process.

Direct nanometer scale patterning of SiO2 with electron‐beam irradiation

Nanometer scale patterns have been fabricated in SiO2 by direct electron‐beam exposure. Two techniques have been developed to eliminate the surface contamination and enable the subsequent development of the patterns in HF based wet etches: (1) exposing the oxide through a sacrificial layer (previously reported) and (2) O2 reactive ion etching (RIE). The latter approach eliminates the need for a sacrificial layer and improves resolution by reducing the forward scattering of the beam. To determine the resolution of this process, patterns were fabricated with both 50‐ and 300‐kV electrons in thin SiO2 membrane samples and imaged in transmission. Transmission imaging avoids the resolution limit of secondary electron micrographs set by the lateral range of secondary electrons. At 300 keV with a line dose of 7.5 μC/cm, arrays of lines with a period down to 15 nm were achieved as opposed to the 21‐nm period previously reported using a sacrificial layer and secondary electron imaging of bulk substrates. A better ...

Nanolithography at 350 KV in a TEM

Abstract The resolution of electron beam exposure of resist has been measured at an accelerating voltage of 350 kV using a modified high voltage transmission electron microscope. The equipment and aspects of its performance are described. The methods developed have been used to fabricate metal nanostructures with dimensions smaller than 10 nm.

A study of the effect of ultrasonic agitation during development of poly(methylmethacrylate) for ultrahigh resolution electron‐beam lithography

Experiments on poly(methylmethacrylate) electron resist have been performed in order to understand how ultrasonic agitation during development affects sensitivity, contrast, and resolution. A modified JEOL 4000 EX electron microscope with a beam diameter of 1 nm was used for writing the resist patterns. First, the development/exposure characteristics of the ultrasonic development process were measured using large area (2×2 μm) patterns. With a 5 s develop in a mix of 3:7 cellosolve:methanol, it was found that ultrasonic agitation increases the effective sensitivity of the resist by roughly 20% but resist contrast does not change significantly (5.7±0.5). It was also found that the sensitivity increases approximately linearly with accelerating voltage from 75 to 400 kV but contrast does not change with voltage significantly. Resolution was explored by writing fine lines at progressively increasing doses in 60 nm of poly(methylmethacrylate) and examining these lines with high resolution scanning electron mic...

Fabrication limits of electron beam lithography and of UV, X -ray and ion-beam lithographies

The paper discusses and compares the lithography methods being developed for the fabrication of future generations of silicon integrated circuits. The smallest features in today’s circuits are about 0.3 μm in size and this will be reduced to 0.1 μm within the next ten years. The methods discussed include optical (ultraviolet light) projection, which is used predominantly at present, projection printing at wavelengths between the X-ray and ultraviolet regions, X-ray proximity printing, and scanning and projection with electrons and ions. There are severe problems to be overcome with all of the methods before they can satisfy future needs. The difficulties are not just connected with obtaining adequate resolution. The more challenging requirements are those associated with the elimination of distortion in the highly complex trillion pixel images and of achieving an exposure rate of about one per second with a system of acceptable cost, that is less than about

Limits of nano-gate fabrication

10M. The various approaches for correcting distortion and obtaining adequate throughput are described, as are the factors limiting resolution. Finally, the ultimate capabilities of electron beam methods for fabricating structures and devices with dimensions down to 1 nm are described.

Characterization of electron beam induced modification of thermally grown SiO2

The authors review the limits of nanometer-scale gate electrode (nano-gate) fabrication. The technology to fabricate nano-gates has become increasingly important in recent years as the scaling limits of conventional electronic devices and the quantum effects of novel devices are investigated. Consistent with the technology used to fabricate virtually all of the smallest devices to date, the emphasis is on the resolution limits of electron beam lithography and associated ultrahigh resolution resists. Recent results of directly patterning SiO/sub 2/ with nanometer-scale resolution by e-beam exposure through a sacrificial layer are also presented. Because the high resistance normally associated with nanometer-scale electrodes seriously limits the performance of high-frequency devices, various techniques to reduce the gate resistance are compared. >

Nanometer scale pattern replication using electron beam direct patterned SiO2 as the etching mask

We used local probe techniques to characterize electron beam (e‐beam) induced changes in thin oxides on silicon. Primary effects of the 1 nm wide, 300 keV e beam included the formation of positive charges trapped in the SiO2, physical restructuring in the oxide, and deposition of carbonaceous compounds. Charges remained stable in thicker oxides (460 nm) and appeared as changes in the contact potential or microwave response with widths down to 100 nm. In thinner oxides (20 nm) the amount of charge was smaller and less stable; below 7 nm no charge was detected. Physical changes in the oxide, evident as a swelling of irradiated areas, accounted for the etching selectivity of these regions.