Electron Image Projector Research Articles

Computer simulation for a large-field electron-beam-reducing image projector

Computer simulation is used to optimize the design of a large-field electron image projector system. Aberrations from mask to target are smaller than 0.02 μm in distortion and 0.12 μm in beam resolution for a 27×27 mm2 field size, 3:1 reduction system, with a 4-m optical path distance. The lenses are given nonuniform bores. The bottom of mask-side lens and the top of wafer-side lens are reduced in diameter to 1/4 of the other bore diameters. The axial direction tolerance for lenses, mask, and target positions is found to be much larger than errors that are anticipated to occur during assembly. The reduction factor can be adjusted to the value designed by adjusting lens positions or mask and target positions, and can be adjusted for the wafer linear distortion through adjusting the lens current ratio of two lenses. We have identified, but not solved, two remaining problems in the lithography system. One is the large emittance electron gun and the other is a condenser lenses spherical aberration.

Ghost proximity correction in an electron image projector

The electron image projector is capable of defining submicron integrated circuit patterns but proximity effects are observed with very fine geometries. The possibility of using ‘‘Ghost’’ proximity corrections has been investigated. It has been shown that features down to 0.3 μm can be reproduced without measurable proximity effects even with fully isolated features.

Proximity corrections for electron image projection

The Electron Image Projector (EIP) is capable of reproducing patterns with dimensions less than one quarter of a micron over a 4″ slice. It has been shown that the requirements for patterning I.C. layers can be met but there was some concern over short range proximity effects. It has now been shown that corrections can eliminate the effects for patterns down to a quarter of a micron. The corrections can be made with one extra noncritical exposure or by using a modified mask. This removes the last remaining obstacle to the use of the Electron Image Projector for defining one quarter micron. I.C.s.

The magnitude and significance of proximity effects in electron image projector defined layers

Electron image projection is an attractive technique for making submicron integrated circuits (IC) but long and short range proximity effects cause some pattern distortion. Long range proximity effects have now been measured and the significance of long and short range effects to IC fabrication assessed. Long range proximity exposure effects due to re‐entrant electrons are sufficiently uniform for local corrections to be unnecessary. With automatic proximity correction IC patterns could be scaled down to below 0.5 μm (1 μm pitch) even with single level resist and 20 keV electrons.

Design and performance of a 4 in. electron image projector

An electron image projector has been designed and built which is currently exposing 4 in. wafers and has the capability of going to 5 in. Linewidth control of ±0.04 μm for 1 μm lines has been measured over the whole surface of 4 in. wafers. Machine alignment accuracy of ±0.03 μm has been achieved and nonrepeatable image distortion is shown to be less than 0.1 μm. Examples of resolution capability and step coverage are also shown.

Processing a conventional magnetic bubble circuit with a 6-µm period

A conventional magnetic bubble memory with a 6-μm period and submicron details has been made. The memory is an 8-kbit shift register single-mask design with field access NiFe propagation elements. The transfer gates and detector area have an 8-μm period, while the major part of the storage loop has an enhanced density with a 6-μm period. The processing is done with a 1:1 electron image projector, which is capable of making the 0.75-μm smallest features necessary for this circuit. The fabrication uses a lift-off technology with Ti followed by a reactive sputter-etch procedure for the structuring of the NiFe elements.

High resolution bubble patterns obtained by electron image projection

The packing density in magnetic bubble circuits is increasing very rapidly. When the smallest dimensions in these circuits are 1 μm or less a successor for the optical lithography becomes necessary. For this purpose an electron image projector is a proper candidate. To study the reliability of the technique for making bubble patterns with this projector a single mask 8 kbit shift register has been realized. The NiFe elements are structured via a Ti lift-off procedure and subsequent reactive sputter etching of the NiFe. The lift-off technology is, among other things favorable owing to the slightly negative slope in the PMMA-resist. A high sputter etch rate ratio between NiFe and Ti during the reactive sputter etching process makes a very thin Ti masking layer feasible. It is shown that bubble devices with smallest features of 1 μm can be made. Uniformity over 2 in. slices is good. Proximity effects are of minor importance.

4100. Recent progress on the electron image projector. (USA)

4100. Recent progress on the electron image projector. (USA)

Recent progress on the electron image projector

Three main aspects of the 1:1 electron image projector are considered; the economics, the ultimate resolution, and an electrostatic chuck. It is shown that the image projector is economic if more than four exposures per mask can be made; the resolution limitation due to the system is shown to be 200–300 Å, but is in practive limited by electron scattering. The effect of electron scattering can be overcome by altering dimensions in patterns down to 1 μm, but isolated windows of 0.5 μm require ’two-mode’ masks. The electrostatic chuck is shown to provide an adequate pressure to hold slices flat, and adequate heat sinking.

Control of pattern dimensions in electron lithography

In electron lithographic processes pattern dimensions are distorted by ’’proximity’’ and ’’size’’ effects due to the action of backscattered electrons. This presents the mask designer with the problem of controlling these dimensions during the mask fabrication process so that the required patterns can be produced on the final substrate. This paper describes the processing of a resolution pattern, minimum geometry 0.5 μm, as it passes through the various stages of mask fabrication to the final stage where it is replicated in an electron image projector. The results show the pattern changes that take place, first at the electron-beam pattern generation stage, second at the mask etching stage, and third at the mask projection stage.

Distortion measurements in an electron image projector

Distortion measurements in an electron image projector

Distortion measurements in an electron image projector

Distortion measurements in an electron image projector for integrated-circuit fabrication are described. It is shown that the major cause of distortion which varies from exposure to exposure is due to slice bowing, and that this distortion is approximately 1/30 the height of the bow. This figure can be reduced by increasing the operating magnetic field strength.

1311. Photocathodes for use in an electron image projector: J P Scott, J Appl Phys, 46 (2), 1975, 661–664

1311. Photocathodes for use in an electron image projector: J P Scott, J Appl Phys, 46 (2), 1975, 661–664

An electron image projector with automatic alignment

An electron image projector for silicon device processing is described which uses an air-cored solenoid and a caesium iodide photocathode. The alignment method is based on the X-rays which are generated when high-voltage electrons strike matter. Phase sensitive detection is used to convert the off balance to a linear signal which is then fed back into analog electronics. Results are presented showing a marker pattern automatically aligned to an accuracy of 0.1 µm.

An electron beam maskmaker

An electron beam machine is described, in which the ¼-µm diameter beam is computer controlled to define integrated circuit and other fine patterns at their final size in response to a coordinate data input. Electron sensitive resist is exposed on metallized quartz or glass substrates. Resist development followed by metal etching enables masks to be made, either for subsequent photolithography or, more usually, for use in the electron image projector developed by J. P. Scott. The mask drawing process is entirely automatic and the emphasis is on the rapid generation of complex patterns with high precision. A two-stage deflection system enables rectangular pattern elements to be drawn at a 10-MHz stepping rate and accurately positioned throughout a 2-mm square main deflection field. Patterns are automatically positioned, to an accuracy of ±1/8 µm, relative to an array of markers predeposited on the substrate. The beam is also refocused automatically at the markers. A mechanical stage for the substrate enables 50 × 50-mm arrays of patterns to be built up. A complete mask containing detail as small as 1 µm takes 1-3 h to draw. Finer patterns can be drawn, although more slowly.

Photocathodes for use in an electron image projector

In this paper factors influencing the choice of photocathode for an image projector are discussed. First of all, a description is given of the principles of operation of an image projector. It is shown that ionic insulators are likely to be the best materials, and the most likely ones are considered. Of these, cesium iodide is the best. In the last section a table is given comparing the observed properties of cesium iodide to those of palladium which has been used up to this time.

LSI Pattern Generation and Replication by Electron Beams

The combined use of the digitally controlled single electron beam pattern generator to make micron-scale LSI patterns and the 1:1 electron image projection system to replicate these patterns on device substrates represents a practical means for realizing high-density integrated circuits in large volume production. After six years of research and development at Westinghouse, most of the obstacles to successful use of these methods have been removed. Among the problems solved are uniform exposure of points, lines, and areas in a device pattern, fabrication of high-resolution electromasks for the electron image projector, and registration of successive device patterns in both electron beams systems, used in combination. In the most recent work, a 1024-bit random access memory was used as a test vehicle. Eight mask levels were involved, including a buried diffusion. In both machines, alignment of a new pattern with a preexisting pattern on the substrate was obtained through electron beam probing of fiducial mark areas. An automatic alignment system developed for the electron image projection system made possible alignment within a few seconds and subsequent controlled exposure. Including alignment errors originating in the electron beam mask generator, over-all pattern registrations were within ±1.5μ.