A Composite Multipurpose l ultipole Electrostatic Optical Structure and a Synthesis Method For Minimizing Aberrations

Technical Field This invention relates to electrostatic optical devices used to control the focusing and deflection of charged particle beams, in particular the invention relates to composite electrostatic structures comprising a series of units having electrodes capable of producing a three dimensional potential function that controls, the focusing and deflection properties and minimizes the aberrations of electron and ion beams in an optical column. The methods relate to designing electrostatic deflectors and focusing lenses and processes using them, more particularly, the method relates to designing electrostatic deflectors and focusing lenses using a synthesis approach, and to process methods that use optical columns having submicron fabrication capability-

Background Art An ideal multipole lens consisting of a number of identical infinite hyperbolic surfaces held at alternate positive and negative potentials is known to provide a solution for the problem of reducing aberrations of charged particle beams. However, since infinite surfaces are not realizable, approximate solutions are required, the most commonly used approximate solution being a number of cylindrical rods. Alternative structures having a greater number of electrodes, i.e. multipole lenses with single plane potential distribution solutions arB known and have been documented by the applicant in Hungarian Patent No. 173.743, issued May 24, 197 .

These multipole lenses are commonly used in particl accelerators? spectrometers and other electron and ion bea devices for focusing, deflection and control of charge particle beams as well as for the correction of aberrations Multipole systems can also stand alone as complete optica columns. Such systems are expected to have lowe aberrations than conventional axially symmetric ones however their extreme complexity requires the overcoming o major design obstacles before being realized. According t the Hungarian Patent No. 173,743, any deflector or multipol lens can be approximated with a high degree of accuracy b a system consisting of an arbitrary number of electrodes o poles arranged around a cylindrical surface and held a suitably chosen potentials, see octupole deflector shown i Fig. 1. This type of structure, although designed having multipole electrode arrangement, does not allow control o the potential distribution along the optical column axis, i.e. in the z direction.

It is also known that a system of short cylinders ca be used for the production of different axially symmetric electrostatic lenses by changing the voltages on th cylinders, see the focusing lens in Fig. 2, showing a serie of twelve (12) non-segmented ring electrodes.

The known methods of designing electrostatic deflector are trial and error analysis methods, whereby a designer uses different geometric arrangements until a satisfactory result is achieved. The deficiencies of adequate reduction of aberrations is typically compensated by using several elements in combination with the designed electrostatic lens or deflector. An octupole deflector, which is known to be a relatively good deflector, can be used as a pre-lenε, post- lense or in-lens deflector to reach a desired deflection sensitivity. The electrode voltages are typically given while the geometric parameters are varied until a satisfactory performance is produced. Typically, there is

very limited play with the parameters of an octupole deflector, the designer may vary one electrical parameter, the length and location of the deflector, until the best result is achieved. This is a fine tuning trial and error approach that does not give a designer control over minimizing the aberrations, hence it is not possible to optimize target spot size resolution and increase the current density of the beam.

The underlying reason for reducing aberrations is that aberrations determine the spot size of a charged particle beam on a given target which in turn relates to a measurement of performance of an optical column. The known electrostatic deflectors can not be controlled to reduce the aberrations of the optical column, hence can not control the spot size nor increase the current density of the beam. This limitation greatly affects productivity in processes involving technologies that are concerned with icrostructures, such as microfabrication of integrated circuits where a charged particle beam must scan the chip surface to write very sharp lines. The integrated circuit industry has constantly been requiring greater and greater performance in methods of writing lines on a chip surface. The chip fabrication industry has evolved from using light beams that write lines on a surface with a resolution not better than 0.5 micrometers, the limitation being due to diffraction, to using electron or ion beams to write lines smaller than 1/2 micron, but only with small current densities, the limitation being due to the problem of aberrations of the optical system. The relationship between spot size, aberrations and current density can be seen from reviewing the following two formulas:

(1) r2 = C (Mr ₀ > ² *+ <dr _g > ² + (dr _c )2 + <dr _d ef1 >2 I) , where r is the spot size radius, M is the magnification of the system, r ₀ is the beam source radius, dr _s is the

spherical aberration disk radius, dr _c is the chromati aberration disk radius and dr^ efl is the deflectio aberration, collectively known as parasitic aberrations.

(2) J = I/( r ^{2 ~} -^ - where J is the current density, r is the spot size radiu and I is the current available from the electron or ion bea source.

By inspection of the two formulas it is seen that reduction of the spot radius is accomplished by reduction o the parasitic aberrations. The spherical and chromatic aberrations can be reduced by the appropriate design of th axially symmetric lens structure, while reduction of the deflection aberrations requires producing a three dimensional potential distribution that is not easily realized using presently known methods. Since the current density is inversely proportional to the square of the spot size radius, any reduction in spot size, i.e., by reducing the aberrations, will result in increase of the current density of the beam. Thus, a need is seen to exist for an electrostatic optical apparatus for use in charged particle beam optical columns that can be controlled to reduce aberrations and optimize resolution and increase current density of the beam to meet the processing needs of modern technologies. A need is seen to exist for providing solutions for processes requiring improved resolution and improved scanning performance to enable the production of design patterns for use in icrofabrication processes, maskless optoelectronic integrated circuit fabrication, microprobe analysis, micro-lithography, maskless implantation doping, microprobe analysis, scanning microscopy, secondary ion mass spectrometry, etching, deposition, micro-machining, mask repair and integrated circuit testing and diagnostics.

A need is seen to exist for a more scientific approach for solving problems of reducing aberrations in optical

columns that involve easily manufactured electrostatic optical elements and that can be controlled to produce a desired three-dimensional potential distribution.

Disclosure of the Invention Accordingly, the primary object of the present invention is to solve these deficiencies by providing a composite lens apparatus that is an easily manufactured geometrical structure that enables a designer to use a synthesis approach to produce a three dimensional potential distribution function that defines the electrode voltages that result in reducing the aberrations.

The composite lens apparatus includes a series of units, that are not discrete lenses, but are simple structures each having multiple electrodes that combine to form a composite lens structure that when coupled to a power supply define a three-dimensional potential distribution function. The shape of the composite lens structure is determined by the geometrical shape of the individual electrodes and the arrangement of electrodes forming the units along the axis of the optical column. The composite lens apparatus may be placed in an optical column as an electrostatic deflector or as a focusing electrostatic lens. The composite lens structure enables scientific determination of an optimized potential distribution within the lens structure for minimizing focusing or deflection aberrations of electron or ion beams in an optical column.

The method is primarily concerned with a procedure that utilizes a synthesis approach for minimizing aberrations in optical columns using the composite lens apparatus of the present invention. The methods of using the composite lens include procedures for use in industrial applications. More specifically, the synthesis approach relates to using the composite lens structure of the present invention and appropriate user system requirements that are mathematically

manipulated using known constrained non-linear optimizatio techniques to arrive at the optimized electrode voltages The voltages are then applied to the electrodes of th composite lens structure to minimize the aberrations an satisfy the system requirements.

Therefore, to the accomplishments of the foregoin objects, the invention consists of the foregoing feature hereinafter fully described and particularly pointed out i the claims, the accompanying drawings and followin disclosure describing in detail the invention, such drawing and disclosure illustrating, however, but one of the variou ways in which the invention may be practiced.

Brief Description of the Drawings

Fig. 1 is a sectional perspective view illustrating a electron or ion beam optical column of the prior ar including an octupole deflector.

Fig. 2 is a perspective view of a prior art ion bea optical column illustrating the focusing electrostatic len as a series of non-segmented rings arranged to produce a axially symmetric potential distribution.

Fig. 3 is a perspective view of a composit electrostatic optical structure of the present inventio illustrating a plurality of easily manufactured disk-lik units formed by multiple discrete electrodes arranged along the optical column axis and provided with electrode voltages that can be controlled to produce a three-dimensional potential distribution that reduces aberrations.

Fig. 4 is a sectional perspective view of an optical column illustrating a rectangular-shaped composite electrostatic deflector and a pair of ring—shaped composite focusing electrostatic lenses according to the present invention.

Fig. 5 is a sectional perspective view of an optical column illustrating an electrostatic deflector and a

focusing electrostatic lens designed according to the composite disk-like structure shown in Fig. 3.

Fig. 6 is a schematic diagram showing an electrically coupled arrangement providing voltages to N different composite electrostatic optical structures from a computer controlled power supply.

Best Mode for Carrying Out the Invention

Before describing the preferred embodiment of the present invention, a description of prior art optical columns 10 and 20 shown in Fig. 1 and Fig. 2 will serve as an introduction. Optical columns for generation of charged particle beams are generally comprised of an electron or ion source 11. Since charged partical beam B directed at target 14 must have accurate spot size S resolution and scanning D characteristics, the design of optical columns includes focusing lenses 12 and deflector 13. The design of electrostatic deflectors, such as the depicted octupole deflector 13, involves trial and error methods before arriving at the depicted shape. Also, the depicted shape is limited in performance due to the complexity associated with changing the potential function Ul , a change that requires a different structure. The potential function Ul is also limited due to the geometric structure, hence no control of aberrations is possible in the z direction. The focusing lens structure 15 depicted in Fig. 2. is an axially symmetric structure with a series of rings that does control the potential distribution in the z direction, but it is limited in that it produces a potential distribution, U2, only in an axially symmetrical manner. Referring now to Fig. 3, where a composite electrostatic structure 40, according to the present invention, is shown comprising a single optical structure L having a series of units 41 formed by an arrangement of electrodes El through En. The embodiment 40 illustrates an

θ easily manufactured geometry and could comprise other composite geometric structures such as deflector LI and focusing lenses L2 and L3 having numerals 31, 32 and 33, as depicted in the optical column 30 of Fig. 4. The intent being to provide a composite structure, such as embodiment 40 that a designer can use in a synthesis approach to produce a three-dimensional potential function LI3 from which the electrode voltages VI through Vn can be determined and applied to satisfy a given set of optical column system design requirements and* constraints and minimize the aberrations.

Fig. 5 illustrates an optical column 50 showing deflector LI and focusing lens L2 adapted according to the disk-like composite embodiment 40 having a three-dimensional potential distribution function U4. It should be appreciated that the plurality of electrodes, E1-E48 for deflector 35 and E49-E136 for focusing lens 36, (E136 being located on unit not shown) offer a high degree of aberration control as the electron beam passes through respective units along the optical column's axis.

Fig. 6 illustrates a schematic diagram of an electrically coupled arrangement between composite lenses Ll-LN and a computer controlled power supply 60 whereby electrode voltages are distributed via interconnecting means 61, 62, and 63. Since the electrode voltages are independently controlled, a single composite optical structure 40 can serve as a deflector and focusing lens. Industrial Applicability

In actual applications, target 34, associated with optical columns 30 or 50, having composite electrostatic structures according to the present invention, are actual surfaces of workpieces W in fabrication processes such as in the production of design patterns for use in microfabrication processes, maskless optoelectronic integrated circuit fabrication, microprobe analysis, micro-

lithography, maskless implantation doping, microprobe analysis, scanning microscopy, secondary ion mass spectrometry, etching, deposition, micro-machining, mask repair and integrated circuit testing and diagnostics. Therefore, while the present invention has been shown and described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is therefore not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent apparatus.