Aaron W. Baum

Semiconductor on glass photocathodes as high‐performance sources for parallel electron beam lithography

The throughput of electron beam lithography has historically been limited by electron–electron interactions that cause blurring at high currents. We present a system configuration for maskless parallel electron beam lithography using a new multiple primary source technology that, by employing widely spaced beams, significantly reduces this problem. The proposed source technology, a negative electron affinity (NEA) photocathode, allows us to generate an array of high brightness, low energy spread, independently modulated beams over a large area. In order to assess the effects of electron–electron interactions in this system, Monte Carlo simulations have been performed. The results of these calculations indicate that this configuration enjoys significant advantages over existing maskless systems. By restricting the area of emission for the individual beamlets to submicron dimensions, the blurring due to statistical electron–electron interactions can be significantly reduced for a given current at the wafer....

Lifetime and reliability results for a negative electron affinity photocathode in a demountable vacuum system

Negative electron affinity photocathodes may have useful applications as electron sources for high-throughput microlithography [A. Baum et al., J. Vac. Sci. Technol. B 15, 2707 (1997)]. However, the nature of such a system has raised questions about the lifetime and reliability of a cathode during operation. In this article, we report on the lifetime and reliability of cathode operation under various conditions applicable to lithography. To perform these measurements, a 632 nm laser was focused onto a spot smaller than 10 μm in diameter on the back surface of the cathode (active area 0.5–2.0 μm thick). The emitted electrons were accelerated to 5 kV to form a magnified image of the cathode on a phosphor screen 1 m away. The 1/e lifetime of the cathode was measured as a function of the cathode current, which turned out to be an inverse relationship. Additionally, a wafer coated with SAL-601 resist was substituted for the phosphor screen to determine if resist outgassing induced by exposure affected operatin...

Scanning electron microscopy of field-emitting individual single-walled carbon nanotubes

Carbon nanotubes are promising electron emitters because of their sharp geometries that lead to significant external field enhancement, as well as their mechanical strength. However, distinguishing the emission due to an individual single-walled carbon nanotube (SWCNT) from that due to surrounding structures is a challenge. Here, we demonstrate how a scanning electron microscope (SEM) can be used to view the emission from individual SWCNTs by applying an external field close to the onset of field-emission and then scanning the tube with the electron beam of the SEM. The stimulated emission is revealed in the SEM image as localized bright spots.

Patterned negative electron affinity photocathodes for maskless electron beam lithography

This work focuses on two issues crucial to achieving high throughput with a negative electron affinity semiconductor photocathode source. Monte Carlo simulations indicate that for a 50 kV system, as much as 8 μA of current may be delivered to the wafer to achieve a raw throughput of 20 8 in. wafers per hour with 0.1 μm minimum feature size (assuming a resist sensitivity of 10 μC/cm2). In order to achieve the throughput potential of this approach, suboptical emission areas are required; this suggests the use of cathode patterning. Two patterning alternatives have been investigated experimentally, and both approaches have been used to generate arrays of more than 100 electron beams with source sizes as small as 150 nm. However, each type of patterned cathode presents unique challenges to fabrication and performance in a practical multibeam system. Different source configurations (number of beams, beam current, beam spacing, etc.) create a system-level tradeoff between resolution and throughput. Results from...

Semiconductor on glass photocathodes for high throughput maskless electron beam lithography

Previous results obtained from negative electron affinity (NEA) photocathodes have shown high brightness (1×108 A/cm2sr at 3 kV), energy spreads as low as 50 meV at room temperature, uniform sensitivity to red and infrared light over cm2 areas, picosecond-scale switching, and low emission noise. These properties make possible a number of new electron beam tools, including a high-throughput multibeam maskless wafer exposure tool. Simulations show that up to 10 μA of current at 50 kV can be delivered to the wafer in such a system with a total spot diameter of 70 nm. The main obstacle to the use of NEA photocathodes in lithography instruments is the issue of cathode stability and lifetime. An ultrahigh system vacuum was built to activate and evaluate NEA photocathodes in a demountable system that includes areas with poorer vacuum. In this system a low quantum efficiency photocathode (<0.1%) was maintained at constant emission up to 190 nA with no discernible decay. A higher-efficiency cathode provided a brig...

Effect of oxygen adsorption on the efficiency of magnesium photocathodes

The quantum efficiency (QE) of photocathodes is key to their being used as electron sources in electron beam lithography and inspection. We have built and are using equipment for measuring quantum efficiency, weight gain, and residual gas pressure. During exposure to oxygen at 10−10–10−8 Torr, we observed an initial rise of between five- and eightfold in quantum efficiency to 0.2% followed by a decrease in QE and saturation of the oxygen uptake (weight gain). At lower oxygen pressures, the cathode exhibits a higher peak QE and larger total weight gain. The peak quantum efficiency can be kept stable to within 1% over 24 h if the ambient pressure is 2×10−10 Torr. The drop in quantum efficiency upon further exposure to oxygen at pressures >10−9 Torr suggests the growth of an oxide layer that reduces emission and the presence of a loosely bound adlayer of molecular oxygen during oxygen exposure.

Negative electron affinity photocathodes as high-performance electron sources-Part 2: Energy spectrum measurements

The energy spectra of electrons emitted from transmission-mode negative electron affinity photocathodes have been measured at high resolution using a parllel-plate retarding technique. The spectra from GaAs photocathodes have a basic structure that varies with temperature, activation layer qualitites, cathode thickness, and illuminating wavelength. A FWHM energy spread of approximately 50meV at room temperature has been achieved. Spectra from a GaAsP cathode show a markedly different structure and a much wider energy spread.

High-performance negative electron affinity photocathodes for high resolution electron beam lithography and metrology

Factors affecting the brightness of negative electron affinity photocathodes have been investigated experimentally and theoretically. The results of a two-dimensional model of electron dynamics in the cathode, including the effects of surface-trapped electrons, are presented. Emission is seen to be cut off in the central emission area at high current levels. The angular distribution is compared to a cosine distribution based on an energy spectrum obtained from the same cathode. While superior to the cosine distribution with respect to its forward focusing properties, it is far wider than previous theory has suggested.