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High Current Electric Scanning Method for Ion Beam Writing

IP.com Disclosure Number: IPCOM000084460D
Original Publication Date: 1975-Nov-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Ko, WC: AUTHOR

Abstract

In introducing ions into a substrate by ion implantation, the dosage depends on two factors: current and time--the higher the current, the shorter the time. In order to decrease the cycles for ion implantation, the art is interested in high-current beams of greater than 0.5 ma.

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High Current Electric Scanning Method for Ion Beam Writing

In introducing ions into a substrate by ion implantation, the dosage depends on two factors: current and time--the higher the current, the shorter the time. In order to decrease the cycles for ion implantation, the art is interested in high- current beams of greater than 0.5 ma.

An unfortunate result of such high currents, is the inability to control the beam spot diameter when the beam is being deflected electrostatically. This is particularly troublesome in ion writing, where the beam diameters are so minute that any significant variation of beam diameters gives rise to critical problems.

It is believed that the undesirable expansion of the beam under high-current conditions, is due to the loss of secondary electrons to the plates of the deflection apparatus. These electrons normally neutralize the beam, and thus prevent the beam from expanding when electrostatically deflected. Because of this shortcoming, at the present state of the art electrostatic scanning has taken a "back-seat" to mechanical deflection when high-current beams are used in ion implantation.

In this description further elaboration on the problems associated with electrostatic scanning with high-current beams will be given, along with, a method of electrostatic scanning which will minimize these problems.

In an unneutralized ion (or electron) beam, the charges in the beam will create a transverse electric field which will cause the beam to expand in diameter. The beam current at which this space-charge effect, which cannot be neglected, is determined by:

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(see, for example, R.G. Wilson and G.R. Brewer, Ion Beams with Application to Ion Implantation, John Wiley, N.Y., 1973, pp. 132 - 143), where V(o) is the energy of the charged particle in volts, m(1) is the mass of the primary ion (or electron), and L and r(o are the propagation length and the radius of the beam, respectively. For example, a 40 KeV arsenic ion beam 0.5 m in length and 2 cm in diameter with current of 70 microamp will be influenced by the space-charge effect.

For beam currents smaller than this value, the space-charge expansion effect can be neglected. This is quite discouraging when trying to achieve a high-beam current to decrease the process time. Fortunately, the ion (or electron) beams are, in general, neutralized by the slow-moving secondary electrons (or ions) generated by the collision of the primary particles with residual gas molecules, or by the impact of the particle on the solid surfaces.

When trying to s...