This invention pertains to the design of a high throughput electron beam lithography system based on an electron beam column equipped with a novel photocathode, which photocathode has a pattern composed of a periodic array of sub-wavelength apertures with a specific geometry for creating a transmission resonance.
As encapsulated by Moore""s Law, there is a longstanding trend in the semiconductor industry toward higher device densities and correspondingly smaller device geometries. One technique for coping with these decreasing device sizes is electron beam lithography in which electrons generated by an electron source are accelerated by an electric field and focused by electron optics onto a semiconductor substrate in order to expose an electron-sensitive resist coating on the substrate surface. Electron beam lithography has the advantage of being able to achieve much higher spatial resolution than light-based lithography.
In a conventional electron beam lithography system, a single beam in a single column is used to expose the resist and create the desired pattern. The throughput of a conventional electron beam lithography system is limited by the total beam current and data delivery rate. As the total beam current is increased, repulsive electron-electron interactions in the beam cause excessive blur, resulting in a degradation of the resolution of the written pattern. Increasing the length of the electron column also tends to increase the blur caused by the electron-electron interactions. Several approaches are known to reduce electron-electron interactions and the associated beam blur. When the total current is distributed evenly among several columns, a reduction in the beam blur of each column results in higher resolution, and if all the columns are operated in parallel, the throughput is not compromised. This is impractical for a conventional column due to its overall dimensions and large footprint. (in addition, the data delivery rate per column is reduced. For a given throughput, multiple beam approaches reduce the incrementing rate by the number of beamlets used.) In a single electron beam column, the total current can also be divided into several beamlets, which approach also reduces the blur due to electron-electron interactions and therefore allows a higher total beam current.
Electron lithography systems may also be divided into those in which one or more electron beams are selectively both modulated and moved over a semiconductor substrate in order to xe2x80x9cdirectly writexe2x80x9d a circuitry pattern in a serial manner without a mask, and those in which an electron emission pattern is created and electron-optically imaged onto a relatively large area of the substrate in order to expose a circuitry pattern in a concurrent manner. Such electron emission patterns may be created by illuminating a photocathode through an optical mask or by using a photocathode that incorporates a mask pattern into its own photoemmissive structure. In the alternative, the electron emission pattern may also be defined by transmitting the electron beam through a mask or reticle.
When photocathodes are used to generate one or more electron beams for use in electron photolithography, it is desirable to be able to fabricate very small emission sites, since this reduces the required demagnification ratio. The optimum demagnification ratio is equal to the ratio of the source angle and the image angle at the substrate. Typical values are 2.5 mrad for the source angle, and 10 mrad for the image angle, which implies an optimum demagnification ratio of xc2xc. Given a spot size of 50 nm at the substrate, and optimum demagnification ratio of xc2xc, the size of the emission site should preferably be approximately 100 nm. Here we assume that the demagnified image and aberrations sum to the total spot size. For larger demagnification ratios, a part of the beamlet current must be cut out by the beam-limiting aperture, and therefore only a fraction of the emitted photoelectrons can be utilized for beam exposure. This requires a larger photoemission current at the photocathode, which increases blur induced by electron-electron interactions and therefore limits achievable throughput. Preferably, the size of the photoemission sites should be less than or equal to 100 nm. However, transmission through apertures substantially smaller than the shortest wavelength of practically available lasers (xcx9c250 nm) is very low. This means that the available photoemission current becomes negligible for such small aperture sizes.
The present invention involves a novel technique using a multiple electron beam column with a wavelength-period matched photocathode pattern for electron beam lithography. The transmission of this periodically patterned photocathode is anomalous, since it acts as an active element with a normalized transmission efficiency larger than 1; i.e., the light transmitted through the aperture is greater than the light directly impinging on it. (As used herein, xe2x80x9clightxe2x80x9d is not intended to be limited to wavelengths in the visible range.) This allows for the efficient illumination of sub-100 nm emission sites, therefore increasing the current available in each individual beamlet. As such, this arrangement allows high throughput to be achieved with very high spatial resolution lithography by simplifying and shortening the column, which reduces significantly the beam blur due to electron-electron interactions.
In a preferred embodiment, the invention is a method and apparatus for implementing sub-100 nm electron beam lithography at high throughput, and with an electron beam lithography column having a simplified design resulting in shorter column length, reduced electron-electron interactions and higher beam current.
In a class of embodiments, the invention is an electron beam lithography system based on an electron beam column having a novel photocathode, which photocathode has a pattern composed of a periodic array of apertures with a specific geometry. The spacing of the apertures is chosen so as to maximize the transmission of a laser beam through apertures significantly smaller than the photon wavelength. The patterned photocathode is illuminated by an array of laser beams to allow blanking and gray-beam modulation of the individual beams at the source level by the switching of the individual laser beams in the array. Potential applications for this invention include direct writing of a circuitry pattern on a target (e.g., a semiconductor wafer) using an electron beam, and mask patterning. In some applications, the electron beam is preferably a high voltage (50-100 kV) electron beam column; however the invention is also applicable to microcolumns operating at lower beam energies such as 1-5 kV.