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