Patent US4283631 - Bean scanning and method of use for ion implantation

FIG.3B X

FIG.3D X— 1 2

deflection, +/—3°, for example, is applied in addition BEAN SCANNING AND METHOD OF USE FOR thereto. The beam experiences a significant variation in ION IMPLANTATION angle of incidence over the extremes of the scan arising

from the deflection process and from simple geometric DESCRIPTION 5 relationships.

1. Field of the Invention It should be fully recognized that it is most often The present invention is in the field of control of desired to distribute the dose over the two-dimensional

scanned beams of charged particles for incidence upon surface of the work piece and to this end the scan will a workpiece and more particularly relates to improve- be two-dimensional, or in the alternative, a onements in the uniformity of dosage or said charged parti- 10 dimensional beam scan will be accompanied by a coopcle beam said workpiece. erative mechanical oscillation of the work piece normal

2. Background of the Invention to the direction of the beam scan. Along the coordinate The irradiation of metals and semiconductor materi- 0f mechanical oscillation the latter hybrid system

als by ion beams provides a means for effecting the avoids the above-discussed non-uniformity in the direc

doping of such materials in a controlled and rapid man- 15 tjon Df mechanical oscillation because the beam is not

ner. Ion implantation, as the process is known, is accom- deflected in that coordinate.

plished by illuminating, for example, a semiconductor A further source of non.uniformity is attributed to wafer with an ion beam of controlled intensity for such the scanning attern and attendant frequency relationintegrated exposure as provides the desired dopant or shipS of the orthogonal deflection means which produce impurity concentration. Uniformity of dopant concen- 20 ^ ^ ^ ... h w0] fee ... ^

tration is a primary quality assurance desiderata. In . , , . . ,. c ,

. . . x ^ i two-dimensional electrostatic scan of an ion beam systems wherein an ion beam is swept at constant trans- , ^ e , ^ , . ,

verse velocity across a planar semiconductor wafer it is *CI0SS a semiconductor wafer is to be corrected for the

known that the resulting areal dose concentration, in- ^cussed non-uniformities One will note that the fre

stantaneous or integrated, will decrease slightly at the 25 quency relationships of the deflection means give rise to

periphery of the scan compared to the concentration Lissajous patterns over full cycles of both deflections,

near the undeflected or central portion of the scan even As the difference in orthogonal scan frequencies in

in the absence of a neutral beam component. This is creases, the average uniformity (or density of scan lines)

purely a geometrical effect arising (in the simplest case) Per unit area improves. In the prior art many full cycles

from the projection upon a plane surface of a uniform 30 of two-dimensional scanning are employed and an an

spherical surface density. More generally, one can pic- harmonic relationship between scan frequencies to

ture the solid angle variation as a consequence of a avoid standing Lissajous figures were thought sufficient

variation in angle of incidence of the beam as it is for irradiation of a work piece.

scanned across the wafer from a fixed center of deflec- One approach to compensating the variation of ion tion. In a typical commercial ion implanter processing a 35 dose with deflection angle is to employ a non-linear four inch diameter wafer, the effect is of the order of waveform which has the effect of varying the transit in density variation across the surface of the wafer. verse beam scan velocity in a desired relationship to the The above described effect is most easily visualized beam deflection angle. Thus, at the extreme deflection wherein the ion beams strikes the central region of the angle the beam scan velocity is decreased to permit the semiconductor wafer or other workpiece at normal 40 accumulation of a larger incremental dose per unit area, incidence and deflection means is employed to scan a An analog approach for complex waveform synthesis beam in a oscillatory fashion across the workpiece. It for this purpose has been attempted wherein break will be clear that the solid angle subtended by a beam of points in the wavef0rm are accomplished by diode and constant cross section varies with the angle of inci- resistor combinations which provide a synthesis of dedence. This picture differs somewhat from actual prac- 45 sired hnear time dependenCes. This method is difficult tice in that normal incidence of the ion beam is ordinar- to accuratel reproduce in a manufacturing environily avo.ded for surfaces of oriented single crystal work- ment and in case> h extremel difficult to adjust pieces in order to thereby avoid crystal channeling

effects which in turn affect the control of the depth of BRIEF SUMMARY OF THE INVENTION

the ion penetration into the substrate. Thus, the plane of 50 T,. . ,. . „ . . .. . ,. ^, , x x , ,• , j, xi. j • x j It is an object ot the present invention to achieve the substrate may be inclined and/or the undeviated . , .„ .. c.r , ■ , c c , j-x- v j * * -i ■ *v. e r improved uniformity of ion dose over the surface of a beam direction may be arranged to strike the surface of ,r Lii JL •, the workpiece at an angle of The order of 6 to 8 degrees Planar substrate scanned by an ,on beam from the normal to the surface. Thus the angular varia- . 11 ,s TMothe* oh^.of the Presf4 invention to synthetion of the angle of incidence will vary over a range 55 Slze a ?esired non-linear waveform by digital circuit which does not include normal incidence if channelling means for compensating non-uniformity in ion dosage is to be avoided. Nevertheless, it will be appreciated of Planar substrates by scanned ion beams, that the variation of solid angle at the target over this In one feature of the Present invention, the scan verange of angular incidence is yet one source of non- loclty rate. transverse to the predominant beam direcuniformity of dosage across the planar workpiece. This 60 tion, is varied systematically by said non-linear waveeffect will be referred to as trace inhomogeneity. form in respect to the displacement of the beam along a Another source of trace inhomogeneity results from coordinate of the planar surface of the substrate the common practice of applying a quiescent deflection wherein the cumulative dose delivered at any location to separate the charged beam from any neutral compo- on the planar surface is substantially independent of the nent. The latter is present due to charge exchange colli- 65 coordinates of such location.

sions of the beam with residual gases in the system. In In another feature of the present invention the scan of

typical apparatus a quiescent 7° offset is applied electro- said ion beam is controlled by a waveform generator

statically to deflect the beam and the periodic scanning wherein said waveform is synthesized from a plurality

of contiguous linear waveform segments, said segments characterized by respective values of slope.

In yet another feature of the invention, orthogonally disposed deflection means produce a non-orthogonal scan pattern, the scan trace and retrace of said scan 5 having an acute angular displacement therebetween.

In yet another feature of the present invention, the frequencies of signals applied to said deflection means are in anharmonic relationships whereby an astable Lissajous pattern is established. 10

In again yet another feature of the present invention the scan pattern is repeated n times and the origin for each repetition is displaced by 1/n AX where AX is a measure of distance between scan lines.

In still again another feature of the present invention, 15 noise in the deflection system is not reduced below a level desired for the further smoothing of the dosage distribution by fluctuation in the path of the scan.

In the present invention uniformity of deposition of ion dose along a given beam trace (trace homogeneity) 20 is accomplished by modulating the rate at which the beam is displaced across the planar surface of the workpiece. The modulating function is synthesized digitally to compensate for the non-linear geometric variation over the workpiece surface of unit solid angle sub- 25 tended thereon by the beam. Planar homogeneity is achieved by a series of scan patterns which distribute the individual scanning trajectories in approximately uniform density over the surface and wherein orthogonal deflection means are adjusted in relative intensity to 30 produce a trace and retrace, each successive trace and retrace forming an acute angle therebetween. The relative frequencies of the deflection means are selected to yield an anharmonic (astable) Lissajous figure which assures a repetitive coverage over the planar area. Pat- 35 terns inherent in the Lissajous distribution are corrected by repeating the planar scan n times with the origin of the scan displaced by a distance 1/n of the interval of reduced (or zero) dose concentration density. Finally, a sufficient amplitude of noise in the deflection system is 40 tolerated in order to provide a further aspect of smoothing of the dosage distribution.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of a typical ion 45 implantation system incorporating the present invention.

FIG. 2 is a block diagram of scanning apparatus of the present invention.

FIGS. 3A-D show the evolution of the scan pattern 50 and the accompanying time base projections.

FIG. 4 shows the geometrical source of non-uniformity.

The present invention in the functional context of an ion implantation system may be seen in FIG. 1. A high voltage terminal 2 is held at high potential relative to ground by high voltage power supply 4. Terminal 2 60 houses the apparatus required to form a beam of ions of desired species. In common practice a gaseous feedstock of the desired species is employed. To this end, a source gas produced from gas handling system 6 is directed to ion source 8. A typical ion source will re- 65 quire a power supply 10 to sustain an ionizing discharge, power supply 12 to impose an axial magnetic field across the discharge region, and extraction supply

14 and vernier 14' cooperating with extraction electrode 16 to shape the electric field at the aperture of the source for effective removal of a well defined high current ion beam. A more detailed description of ion source techniques is outside the scope of this work. See for example L. Valyi, "Atom and Ion Sources, WileyInterscience, 1978." The beam 18 diverging from the ion source 8 is momentum analyzed in analyzer magnet 20, the latter energized from analyzer power supply 22. Analyzed beam passes through analyzer exit slit 24 and then to accelerator tube 26 where it encounters a carefully designed field gradient from the high voltage terminal 2 to ground potential. Optical elements, such as a quadrupole triplet 28, 30, 32, and associated control system 34, operate to produce a spatial-energy focus at a desired image plane. Two sets of electrostatic deflection plates, 40 and 42, arbitrarily labeled y and x respectively serve to direct the beam over the desired area of the image plane. The waveform applied to the respective deflection plates and their synchronization to form the appropriate scanning program is accomplished by scanning system 43. The quiescent beam is deflected sufficiently to completely separate neutral beam 44 (arising from charge-exchange collisions with residual gases) from the charged beam. Target chamber 46 contains beam defining apertures, beam monitoring and integrating apparatus and equipment for introducing the wafer substrate in the vacuum system and aligning same with respect to the target plane.

Vacuum pumping apparatus and the vacuum envelope of the system is not indicated but it will be appreciated that the entire region traversed by the beam is maintained at high vacuum.

In a commercial context it is extremely important that a high degree of uniformity of ion dosage be maintained over the surface of the wafer and that the time required per wafer be minimal. Accordingly, it was determined that the implant time per wafer be less than 10 seconds and this parameter limits the scan program performed by scanning system 43 and the attendant frequencies of waveforms selected by application deflection plates. Additionaly, the frequencies are anharmonic with respect to the 50 or 60 Hz commonly employed in power transmission system.

The scanning system of the preferred embodiment is now described with the aid of FIG. 2. An anharmonic frequency ratio of 200:23 was selected for the scanning frequencies in the x and y directions. In the present work, the x coordinate is understood to refer to the coordinate in the plane of the quiescent offset deflection and orthogonal to the undeflected beam (z axis). A master clock 60 supplies a train of 3.00 MHz pulses to dividers 62 and 63. These dividers supply respectively 8152 Hz pulses to the x scan generator 70 and 117 Hz pulses to y scan generator 71. In the x scan generator 70, the 8152 Hz pulse train is split into 2 additional channels. One such channel provides an additional division by 8 to produce 1019 Hz pulses for the fundamental x scanning frequency. A second division of the 8152 Hz input by a factor 1024 results in an 8 Hz output for use discussed later below. The trace inhomogeneity compensation is obtained in the manner outlined by tailoring the waveform to the desired non-linear shape whereby the rate at which the beam is deflected is controlled. In the present invention it is recognized that a non-linear shape will be synthesized from a plurality of linear segments. To that end x scan generator 70 comprises a triangle wave generator 72 which drives scan amplifier

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Bean scanning and method of use for ion implantation