WO2002054442A2

WO2002054442A2 - Ion beam collimating system

Info

Publication number: WO2002054442A2
Application number: PCT/US2001/049866
Authority: WO
Inventors: Harald Enge
Original assignee: Proteros, Llc
Priority date: 2000-12-28
Filing date: 2001-12-28
Publication date: 2002-07-11
Also published as: AU2002239682A1; WO2002054442A3

Abstract

An ion beam collimation system is used in ion implantation equipment to focus the beam and render the beam in parallel. A magnetic lens assembly produces these results by the action of opposed magnetic plates (214, 216) that produce magnetization vectors (204, 206) or opposite orientation. The magnetic field within a gap between the two plates varies linearly with distance from a central point (229) of neutral magnetic field.

Description

ION BEAM COLLIMATOR SYSTEM RELATED APPLICATIONS

This application claims benefit of priority to provisional application serial number 60/258,844 filed December 28, 2000, which is hereby incorporated by reference to the same extent as though fully repeated herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the field of particle accelerators and, more particularly, ion beam generators that may be used in ion implantation equipment for semiconductor manufacturing. Still more specifically, the ion implantation equipment is able to focus and render a scanned ion beam into a parallel configuration.

2. Description of the Related Art

Semiconductor manufacturing processes use ion implantation equipment to generate and embed ions in semiconductor devices, such as silicon wafers. In ion implantation, a beam of energetic ions - impinges upon a surface of material to imbed or implant those ions into the material. Ion implantation processes are categorized into batch and serial processes. Serial processes are the most common type of ion implantation processes, and are associated with medium dose implantation. Serial processes most often use a plasma ion beam that is subjected to electrostatic deflection processes in both axes normal to the direction of beam propagation. The electrostatic deflection processes are intended to provide a uniform distribution of ions in terms of density and direction of travel, but in practice many ion beams usually vary in angle by about 3° or more relative to the direction of beam propagation. This variance produces undesirable effects in the ion implantation processes, as reported in United States Patent No. 4,726,689 to Pollock.

United States Patent 5,350,926 to White et al. describes a high current broad beam ion implanter with emphasis upon systems for beam control to establish uniformity across a large ribbon shaped beam traveling in a single transverse direction. The ion implanter uses a Freeman, Bernas, or microwave source, from which the ion beam is extracted from source plasma through a parallel-sided convex slot. The ion beam passes through a pair of analyzing magnets to render the beam parallel in both axes normal to the direction of beam propagation. United States Patent No. 4,922,106 to Berrian et al. similarly shows an ion beam implantation device having a parallel beam generator together with mechanical and electrical scan controls that facilitate uniform implantation. United States Patent No. 4,276,477 to Enge, which is hereby incorporated by reference to the same extent as though fully replicated herein, describes the use of paired sector magnets in focusing the scanned ion beam. The sector magnets each contain shaped ferromagnetic material within an electrical coil. Similarly, United States Patent No.4,745,281 to Enge, which is also incorporated by reference to the same extent as though fully replicated herein, describes the use of a dipole magnetic lens that is formed of shaped magnetic elements each having a wedge configuration. The wedge configuration is intended to overcome fringe field effects that, otherwise, would distort the ion beam if the magnetic lens were constructed in a non-wedge configuration.

As exemplified by differences in the Enge patents, it is generally accepted that shaped magnetic elements are needed to reduce fringe field effects that distort ion beams. In practice, the fixed nature of these shaped elements do not meet the demands for all process conditions to which an ion implanter will be subjected, and it is desirable to find an alternative structure that is adaptable to a wider range of process conditions.

Another problem in the art is that target sizes are increasing. United States Patent Nos.5,406,088 and 5,229,615 to Brune et al. describe a parallel beam ion implantation device that was developed in response to increasing commercial use of large wafer diameters. The growth in wafer diameter from 4 " to 6" and then to 8" in diameter has generated a need for a serial implantation device capable of producing a beam that strikes the surface of the wafers with a uniform parallel beam while also permitting tilt and rotational control of the wafers. Increased target sizes require the generation of broader ion beams, which entail beam deflection to flatten and spread out the ion beam into a fan configuration that strongly diverges in a plane. This type of broad beam is more difficult to focus and render parallel.

SUMMARY OF THE INVENTION The present invention overcomes the problems outlined above by providing a new type of ion beam collimator system that uses opposed magnetic plates, as opposed to problematic shaped elements, and is readily adaptable ion implantation processes that operate upon broad beams that strongly diverge in a plane.

The ion beam collimator system operates on a scanned ion beam that travels along a beam pathway. An ion beam source produces the ion beam, and a beam deflector produces a scanned ion beam such that the scanned ion beam assumes a flattened fan configuration in which ions in the beam travel in nonparallel paths along the beam pathway. A magnetic quadrupole lens system focuses the scanned ion beam and renders the scanned ion beam into a parallel configuration along the beam pathway. The quadrupole lens system comprises a first ferromagnetic plate that is magnetized by current in a first electric coil wound around the first ferromagnetic plate to produce a first magnetization vector. A second ferromagnetic plate is magnetized by current in a second electric coil wound around the second ferromagnetic plate to produce a second magnetization vector. A gap exists between the first ferromagnetic plate and the second ferromagnetic plate to permit passage of the ion beam through the gap. The first magnetization vector and the second magnetization vector have opposed pole orientations with respect to one another. By symmetry in the magnetic field, the gap contains a point or line of neutral magnetic field. The first and second magnetic vectors are, in preferred but optional embodiments, adjusted or tuned such that net magnetic field in the gap increases linearly with distance from the point of neural magnetic field.

As used herein, the term "opposed pole orientations" is defined to mean that the north pole of the first ferromagnetic plate resides over the south pole of the second ferromagnetic plate, and the south pole of the first ferromagnetic plate resides over the north pole of the second ferromagnetic plate. Accordingly, the first magnetization vector and the second magnetization vector may have directional orientations that are exactly opposite one another, or the directions may differ by an angle. This angle may comprise a small deviation from an exact opposite orientation, e.g., on the order from 170° to 190° degrees of arc, but the angle may be any angle from 90° to 270°. In the most preferred configuration the orientations of both the first magnetization vector and the second magnetization vector are perpendicular to the beam pathway. A mechanism may be provided to adjust the angular orientation of the first and second ferromagnetic plates, in order to fine-tune the magnetic lens assembly.

The term "plate" may be defined as an object having thickness, width and depth dimensions such that the width and depth dimensions are much larger than the thickness, such as four, eight or ten times larger.

In one aspect, the ion beam collimator system operates according to a method comprising the steps of generating an ion beam, deflecting the ion beam to produce a scanned ion beam such that the scanned ion beam assumes a flattened fan configuration in which ions in the beam travel in nonparallel paths along the beam pathway; and rendering the scanned ion beam into a parallel configuration by passing the scanned ion beam through a gap between a first ferromagnetic plate and a second ferromagnetic plate. The step of rendering the scanned ion beam into a parallel configuration comprises magnetizing the first ferromagnetic plate to produce a first magnetization vector, magnetizing the second ferromagnetic plate to produce a second magnetization vector wherein the first magnetization vector and the second magnetization vector have substantially opposed pole orientations with respect to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram that depicts an ion beam collimator system incorporating a magnetic lens assembly;

Fig.2 is a midsectional view of the magnetic lens assembly;

Fig. 3 is a schematic diagram representing the effect of the magnetic lens assembly upon an ion beam; and

Fig.4 is a plot of distance versus magnetic field showing a linear field effect within the magnetic lens assembly.

DETAILED DESCRIPTION

Fig. 1 is a block diagram showing a top plan view of an ion beam collimator system 100 that demonstrates the relationship between various system components according to the instrumentalities described above. The various components of Fig. 1 are intended to demonstrate one embodiment of an ion implantation device that may utilize the collimator system and should not be construed in a manner that is unduly limiting.

The ion source 102, analyzer magnet 104, image slit 106, deflector 108 and accelerator 110 are conventional components of ion implantation systems. The ion source 102 may be a conventional a

Freeman, Bernas, or microwave source, from which an ion beam 112 is slot-extracted from source plasma. The ion beam 112 passes through the analyzer 104 for particle selection and mass control purposes, and through the image slot 106, which provides an initial beam shaping function. Other devices, such as a deflector 108 may be provided to scan the ion beam and render the same, for example, into a broadly diverging flattened fan shape 112A that includes ions traveling in non-parallel directions. A magnetic lens assembly 114 focuses and renders the ion beam 112A into a parallel orientation 112B, which is accelerated by the accelerator 110 for impingement upon a target 116, such as a silicon wafer.

Fig. 2 is a midsectional view of the magnetic lens assembly 114 taken on the median plane of magnetic lens assembly 114. Fig.2 is also taken along a perpendicular to arrow 118 (see Fig. 1), which represents a direction of ion beam propagation along the ion beam pathway from the ion source 102 to the target 116. The magnetic lens assembly 114 includes a first magnetic member 200 and a second magnetic member 202. As shown in Fig.2, the first magnetic member 200 is identical to the second magnetic member 202, except that the first magnetic member produces a first magnetization vector 204 that is aligned in an opposite direction with respect to a second magnetization vector 206 which is produced by the second magnetic member 202. The first magnetic member 200 and the second magnetic member 202 have opposed magnetic orientations, such that a north pole 208 of the first magnetic member 200 resides over a south pole 210 of the second magnetic member 202, and a south pole 212 of the first magnetic member 200 resides over a north pole 214 of the second magnetic member. The magnetic field indicated by lines 224 and 226 is a quadrupole field that varies accordingly off the median plane.

The first magnetic member 200 is made of a first ferromagnetic plate 216 that is wound with a first electrical coil 218. The second magnetic member 202 is made of a second ferromagnetic plate 220 that is wound with a second electrical coil 222. Electrical current on coils 218 and 222 produces a magnetic field, as indicated by lines 224 and 226, within a gap 228 that separates the first magnetic member 200 from the second magnetic member 202.

A central line or point 229 at the center of the magnetic lens assembly 114 represents a point of zero field by symmetry. Field in the direction of magnetization vectors 204 or 206 increases linearly with distance from point 228.

Fig.3 schematically represents the magnetic lens assembly 114 in operation on ion beam 112A. The ion beam 112 A contains various particles that travel on paths such as paths 300 and 302. On average, these particles travel in the direction of arrow 118, but the individual paths 300 and 302 differ from directions in parallel with the direction of arrow 118, for example, as angle β in the case of path 302. The action of magnetic lens assembly 114 reduces or eUminates the deviation from parallel because the field effects within gap 228 drive the ion particles towards a parallel path of travel. An angle θ taken at a point on path 302 between a line 304 that is parallel to line 118 and a tangent 306 to path 302 is preferably reduced to zero by the action of magnetic lens assembly 114; however, it is acceptable that the magnetic lens assembly provides an angle θ that is less than angle β. The individual particles in ion'beam travel in directions that, in preferred but optional embodiments, deviate from one another by no more that about 0.3°; however, the instrumentalities described above may be used to obtain any tolerance that is useful in parallel ion beam implantation processes. Fig. 4 represents calculation from a two dimensional analytical computer program POISSON, which is available upon request from the Los Alamos Accelerator Code Group of the Los Alamos National Laboratories located in Los Alamos, New Mexico. The curve shows the horizontal field in the median plane of the magnetic lens assembly 114 for the right handed half of the magnetic lens assembly 114 shown in Fig. 2. These calculation results show that the field increases linearly (i.e., substantially linearly) with distance from point 229. The linear portion 400 exists for about three-fourths of the distance, while the curved region 402 exists due to fringing effects. In practice, the ferromagnetic plates 216 and 220 have a finite width that is large compared to the dimension of gap 228, so that the fringing field effects do not substantially vary the magnetic field within gap 228. The calculation results shown in Fig.4 represent those for a magnetic lens assembly 114 having iron plates with a distance of six centimeters between the plates across gap 228 and a total plate length of forty centimeters.

The calculation results shown in Fig.4 would apply in an identical manner for all points at any distance taken as a perpendicular to the plane shown in Fig.2. A fine tuning mechanism is provided that permits adjustment of the magnetic field in gap 228, as indicated by arrows 118 and 120 in Fig. 1. The first and second magnetic members 200 and 202 (see Fig.2) may be rotated within their existing planes to skew the opposite orientations of the first and second magnetization vectors 204 and 206. This rotation provides a fine tuning mechanism for adjusting the focus and parallel nature of ion beam 112. Small angular rotations of less than 10° in arc are most likely all that is needed, such that the directional orientation of first magnetization vector 204 may differ from the orientation of second magnetization vector 206 by from 170° to 190°, as opposed to being exactly opposed at 180°. Rotations of any degree of arc may be performed, for example, such that the difference in directional orientation ranges from 90° to 270°.

The invention in its broader aspects is not limited to the specific details, representative devices and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

CLAIMSWe claim:

1. An ion beam collimator system for use with a scanned ion beam that travels along a beam pathway, comprising: an ion beam source for use in producing an ion beam; a beam deflector operable to produce a scanned ion beam such that the scanned ion beam assumes a flattened fan configuration in which ions in the beam travel in nonparallel paths along the beam pathway; and a magnetic quadrupole lens system operable to focus the scanned ion beam and to render the scanned ion beam into a parallel configuration along the beam pathway, the quadrupole lens system comprising a first ferromagnetic plate magnetized by current in a first electric coil wound around the first ferromagnetic plate to produce a first magnetization vector, a second ferromagnetic plate magnetized by current in a second electric coil wound ^" around the first ferromagnetic plate to produce a second magnetization vector; a gap between the first ferromagnetic plate and the second ferromagnetic plate permitting passage of the beam pathway through the gap, the first magnetization vector and the second magnetization vector having opposed pole orientations with respect to one another.

2. The ion beam collimator system as set forth in claim 1, wherein the gap contains a point of neutral magnetic field.

3. The ion beam collimator as set forth in claim 1, wherein the magnitude and orientation of the first magnetization vector and the second magnetization vector are such that net magnetic field in the gap increases linearly with distance from the point of neural magnetic field.

4. The ion beam collimator system as set forth in claim 1, wherein the opposed pole orientations between the first ferromagnetic plate and the second ferromagnetic plate are such that the first magnetization vector and the second magnetization vector have opposite directions.

5. The ion beam collimator system as set forth in claim 4, where the orientations of the first magnetization vector and the second magnetization vector are peφendicular to the average moment of the beam pathway.

6. The ion beam collimator system as set forth in claim 1 comprising a mechanism for adjusting the orientation of the first magnetic vector and the second magnetic vector to provide fine tuning adjustments useful in rendering the scanned ion beam into the parallel configuration.

7. A method of collimating a scanned ion beam, the method comprising the steps of: generating an ion beam; deflecting the ion beam to produce a scanned ion beam such that the scanned ion beam assumes a flattened fan configuration in which ions in the beam travel in nonparallel paths along the beam pathway; and rendering the scanned ion beam into a parallel configuration by passing the scanned ion beam through a gap between a first ferromagnetic plate and a second ferromagnetic plate while magnetizing the first ferromagnetic plate to produce a first magnetization vector, magnetizing the second ferromagnetic plate to produce a second magnetization vector wherein the first magnetization vector and the second magnetization vector have opposed pole orientations with respect to one another.

8. The method according to claim 7, wherein the steps of forming the first magnetization vector and forming the second magnetization vector comprise forming a magnetic field that increases linearly with distance from a point of neutral magnetic field within the gap.

9. The method according to claim 7, wherein the step of magnetizing the first ferromagnetic plate comprises applying electrical current to a first coil wound about the first ferromagnetic plate.

10. The method according to claim 9, wherein the step of magnetizing the second ferromagnetic plate comprises applying electrical current to a second coil wound about the second ferromagnetic plate.