Abstract
A new application of superconducting magnet has been opened out for a microfabrication system in future semiconductor itegrated circuits industry. Current interests are to develop electron beam pattern transfer systems which have capability of submicron patterning. A photocathode pattern transfer system has been developed in VLSI Cooperative Laboratories. In this system photoelectrons emitted from a mask with submicron VLSI patterns are accelerated by high electric field and are focused onto a silicon wafer by the highly homogeneous magnetic field.
It is possible to transfer VLSI patterns onto many silicon wafers in turn with high resolution by use of this pattern transfer system. The system is superior to conventional optical projection system in regard to resolution capability.
The requirements for the magnet in the system are as follows;
(1) Magnetic field intensity is around 3k Gauss at the center.
(2) Field homogenity is of 10-5 order over 90mm in diameter at the central part.
(3) Stability of magnetic field is less than 10-4/hour.
(4) The room temperature region for working space is more than 500mm in diameter.
A superconducting magnet is suitable for above requirements because of its characteristics such as persistent current and high current density. Stable magnetic field will be obtained by persistent current mode operation. A Helmholtz coil with high current density will easily provide homogeneous magnetic field.
The magnet developed here is of Helmholtz type. Each coil has the average diameter of 880mm and the winding crosssection of 28mm×28mm. The maximum current density over the winding crosssection is 188A/mm2. The upper and lower coils are enveloped in each cryostat and are connected by superconducting wire for persistent current mode operation. The cryostat with 620mm bore in diameter was designed in consideration of wide space for the components of pattern transfer equipments.
The magnet was successfully operated in persistent current mode up to the field of 3k Gauss. Adjustment of coil separation was made to get the field homogeneity, measuring the field distribution by NMR method. Miscellaneous magnetic field due to magnetization of surrounding materials or relative displacement of two coils was fairly well cancelled out by the adjuster. As a result, 6×10-5 field homogeneity was achieved over 90mm in diameter at the center. Superconducting persistent current was kept with 10-4/hour in decreasing ratio which was enough for stability requirement.
Consequently VLSI pattern resolution with less than 0.5μm geometry was successfully obtained. The superconducting magnet would give a promising result to realize VLSI like 1M bits Random Access Memory (RAM).
