Abstract:
A dynamic surface anneal apparatus for annealing a semiconductor workpiece has a workpiece support for supporting a workpiece, an optical source and scanning apparatus for scanning the optical source and the workpiece support relative to one another along a fast axis. The optical source includes an array of laser emitters arranged generally in successive rows of the emitters, the rows being transverse to the fast axis. Plural collimating lenslets overlie respective ones of the rows of emitters and provide collimation along the fast axis. The selected lenslets have one or a succession of optical deflection angles corresponding to beam deflections along the fast axis for respective rows of emitters. Optics focus light from the array of laser emitters onto a surface of the workpiece to form a succession of line beams transverse to the fast axis spaced along the fast axis in accordance with the succession of deflection angles.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/508,781, filed Aug. 23, 2006 now U.S. Pat. No. 7,674,999 entitled FAST AXIS BEAM PROFILE SHAPING BY COLLIMATION LENSLETS FOR HIGH POWER LASER DIODE BASED ANNEALING SYSTEM, by Dean Jennings, et al. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to thermal processing of semiconductor substrates. In particular, the invention relates to laser thermal processing of semiconductor substrates. 
     BACKGROUND ART 
     Thermal processing is required in the fabrication of silicon and other semiconductor integrated circuits formed in silicon wafers or other substrates such as glass panels for displays. The required temperatures may range from relatively low temperatures of less than 250 degrees C to greater than 1400 degrees C and may be used for a variety of processes such as dopant implant annealing, crystallization, oxidation, nitridation, silicidation, and chemical vapor deposition as well as others. 
     For the very shallow circuit features required for advanced integrated circuits, it is greatly desired to reduce the total thermal budget in achieving the required thermal processing. The thermal budget may be considered as the total time at high temperatures necessary to complete device fabrication. The time that the wafer needs to stay at the highest temperature can be very short. The greater the total time that the wafer is subject to high temperatures, the more features such as implanted junctions will diffuse and loose their definition. For example, implanted junction depths may become deeper than desired due to diffusion. 
     Rapid thermal processing (RTP) uses radiant lamps which can be very quickly turned on and off to heat only the wafer and not the rest of the chamber. Pulsed laser annealing using very short (about 20 ns) laser pulses is effective at heating only the surface layer and not the underlying wafer, thus allowing very short ramp up and ramp down rates. 
     A more recently developed approach in various forms, sometimes called thermal flux laser annealing or dynamic surface annealing (DSA), is described in U.S. Pat. Nos. 7,005,601 and 6,987,240, the disclosures of which are incorporated herein by reference. The DSA system employs many CW diode lasers focused on an extremely narrow (0.07 mm) line beam to produce a very intense beam of light that strikes the wafer as a thin long line of radiation or line beam. The line beam is then scanned over the surface of the wafer in a direction perpendicular to the long dimension of the line beam. 
     The thinness of the line beam (e.g., 0.07 mm) ensures very short temperature rise and fall times and a short dwell time at the required temperature, e.g., 1300 degrees C, with respect to a fixed location on the wafer surface that is scanned once by the line beam. For example, the temperature of a fixed location on the wafer surface will increase from an ambient 450 degrees C to 1300 degrees C within 0.6 ms, assuming a scan rate within a range of about 50-300 mm/sec is employed. The advantage is that the wafer surface spends an extremely short amount of time at lower or intermediate temperatures (e.g., 500-900 degrees C) at which the higher silicon thermal conductivity promotes heating throughout the wafer and consequent diffusion and loss of underlying circuit feature definition. Instead, the wafer surface spends more time at the desired high temperature (e.g., 1300 degrees C) at which silicon thermal conductivity is lowest for minimum heating of the underlying features, and at which desired effects are maximum (e.g., annealing of implanted dopant impurities, annealing of pre-implant amorphization damage, etc.). The thinness of the line beam corresponds to the minimum resolvable spot size R of the laser beam optical system, which is governed by the following approximate formula:
 
 R=λ/ 2  NA ,
 
where λ is the laser wavelength and NA is the numerical aperture of the optics. Numerical aperture is defined as:
 
 NA=n  sin θ/2,
 
where n is the index of refraction and θ is the angle subtended by the beam between the aperture at the lens and the focal point in a simple or ideal system. In the DSA system referred to above, the wavelength is 810 nm and the angle θ is less than 60 degrees and n is the index of refraction of air (about 1).
 
     These parameters provide a minimum resolvable spot size R corresponding to the small width of the line beam (0.07 mm). Within the preferred beam scanning rate range (50-300 mm/sec), each fixed wafer surface location spends less than about 0.5 ms near the peak temperature (e.g., 1300 degrees C). 
     The required level of the wafer surface peak temperature (1300 degrees C) requires a power density of about 220 kiloWatts/cm 2 . To reach this level, the DSA system employs a large number of 810 nm CW diode lasers focused on the same line beam image, as will be described later in this specification. 
     One problem recently encountered is that some annealing processes requires a longer time at or near the peak temperature (1300 degrees C), than the current dwell time of less than 0.5 ms. This dwell time may be sufficient to cause ion implanted dopant impurities to become substitutional in the semiconductor crystal lattice. However, it may be insufficient to completely cure pre-ion implantation amorphization defects. Pre-ion implantation amorphization is performed prior to ion implantation of dopant impurities to form shallow PN junctions to prevent channeling of the kinetic dopant ions through the crystal lattice below the desired junction depth. Amorphization prevents such channeling by ion bombardment of the wafer with heavier atomic species (oxygen, nitrogen, carbon), causing bombardment damage to at least partially convert the active semiconductor layer from a crystalline state to an amorphous state. The defects in the crystal are cured provided each wafer surface location has a sufficiently long dwell time near 1300 degrees C This may require a dwell time that is longer than the current 0.5 ms dwell time. Furthermore, conversion of the amorphized region back to a crystalline state essentially forms an epitaxial crystalline layer over the bulk crystalline layer, giving rise to another class of defects, namely boundary defects at the interface between the bulk crystal and the re-crystallized surface. 
     Such boundary defects have been found to be more persistent than the other types of defects, and require a significantly longer dwell time to completely cure or remove, as long as 2 to 3 ms near 1300 degrees C. 
     In order to provide such a long dwell time, the beam spot size must be enlarged, which essentially broadens the Gaussian profile of the beam intensity along the direction of scan, hereinafter referred to as the “fast axis”. Unfortunately, if the Gaussian beam profile is widened by a given factor, then the slope of the leading edge of the Gaussian beam profile is reduced by approximately the same factor. This increases the temperature rise time (and fall time), thereby subjecting each location on the wafer surface to a longer time at a lower or intermediate temperature, and thus degrading the device structure through thermal diffusion, for example. Thus, widening the beam profile along the fast axis does not appear to be possible. Therefore there is a need for a DSA process in which the boundary defects and other defects in an ion implanted wafer can be annealed without device degradation due to greater thermal diffusion. 
     SUMMARY OF THE INVENTION 
     A dynamic surface anneal apparatus for annealing a semiconductor workpiece has a workpiece support for supporting a workpiece, an optical source and scanning apparatus for scanning the optical source and the workpiece support relative to one another along a fast axis. The optical source includes an array of laser emitters arranged generally in successive rows of the emitters, the rows being transverse to the fast axis. Plural collimating lenslets overlie respective ones of the rows of emitters and provide collimation along the fast axis. The selected lenslets have one or a succession of optical deflection angles corresponding to beam deflections along the fast axis for respective rows of emitters. Optics focus light from the array of laser emitters onto a surface of the workpiece to form a succession of line beams transverse to the fast axis spaced along the fast axis in accordance with the succession of deflection angles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an orthographic representation of a thermal flux laser annealing apparatus employed in the present invention. 
         FIGS. 2 and 3  are orthographic views from different perspectives of optical components of the apparatus of  FIG. 1 . 
         FIG. 4  is an end plan view of a portion of a semiconductor laser array in the apparatus of  FIG. 1 . 
         FIG. 5  is an orthographic view of a homogenizing light pipe for the apparatus of  FIG. 1 . 
         FIG. 6  is a schematic diagram corresponding to  FIGS. 2 and 3 . 
         FIG. 7  is a perspective view of a portion of the beam source of an embodiment of the invention. 
         FIG. 8  is an end view corresponding to  FIG. 7  and further illustrating a controller for varying the fast axis power density profile of the two line beams. 
         FIG. 9  is a graph depicting the fast-axis power density profile obtained with the apparatus of  FIGS. 7 and 8 . 
         FIG. 10  is a schematic diagram of another embodiment of the invention employing several displace Gaussian beams with a programmable fast-axis profile. 
         FIGS. 11A through 11E  are graphs of different fast-axis power density profiles that can be selected with a programmable controller of the DSA apparatus of  FIG. 10 . 
         FIG. 12A  is a graph depicting a preferred power density profile over time generated with the DSA apparatus of  FIG. 10 . 
         FIG. 12B  is a graph contemporaneous with the graph of  FIG. 12A  and depicting the time behavior of the temperature of a fixed spot on the wafer surface resulting from the power density profile of  FIG. 12A . 
         FIG. 13  is a cut-away side view of a semiconductor device formed using the DSA apparatus of the invention. 
         FIG. 14  depicts a DSA process employing the apparatus of  FIG. 10 . 
         FIG. 15  is a schematic diagram depicting a first modification of the embodiment of  FIG. 8  employing beam deflecting mirrors. 
         FIG. 16  is a schematic diagram depicting a second modification of the embodiment of  FIG. 8  employing a single beam deflecting mirror. 
         FIGS. 17 and 18  depict a third modification of the embodiment of  FIG. 8  in which selected ones of the fast-axis collimating cylindrical lenses are rotated through the desired beam-deflecting angle. 
         FIG. 19  is a schematic diagram depicting a first modification of the embodiment of  FIG. 10  in which the prisms of successively greater beam deflection angles are replaced by rotating the corresponding fast-axis cylindrical lenses through successively greater angles. 
         FIG. 20  is a schematic diagram depicting a second modification of the embodiment of  FIG. 10  in which the prisms of successively greater beam deflection angles are replaced by respective beam deflecting mirrors rotated through successively greater angles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Introduction: 
     Defects in an ion implanted wafer of the type that persist beyond a half millisecond at high temperature (1300 degrees C) are annealed or cured by providing a very long dwell time (e.g., 2-3 milliseconds) without compromising the extremely steep rising and falling edges of the Gaussian shaped line beam along the fast axis. In this way, boundary defects that are an artifact of pre-implant amorphization and post implant annealing are completely removed without incurring a corresponding penalty in thermal diffusion. All this is accomplished by focusing a first set of lasers on a first line beam image and focusing a second set of lasers on a second line beam image whose amplitude peak is displaced from the amplitude peak of the first line beam image along the direction of the fast axis (i.e., perpendicular to the length of the line beam). This displacement is preferably the width of the Gaussian profile of the line beam along the fast axis at an amplitude corresponding to half the peak amplitude of one of the two line beams. Both line beams have the same highly focused image with the minimum resolvable spot size of about 0.07 mm as before. The angle subtended between the optical paths of the two sets of lasers is less than one degree in order to achieve such a small displacement, and this angle depends upon the distance between the lasers and the wafer surface. 
     The net effect is the same extremely steep slope of the leading and trailing beam edges along the fast axis as with a single line beam, but with a dwell time at or near the peak temperature that is doubled from that of a single line beam. One of the laser line beams is the leading beam while the other is the trailing beam. The leading beam must raise the wafer surface temperature from about 400 degrees C to 1300 degrees C within 0.5 ms, and must therefore be of higher power density, while the trailing beam must simply maintain the wafer surface at, the elevated temperature (without increasing the temperature), and is therefore of a lesser power density. 
     The time-profile of the wafer surface temperature at each fixed location may be adjusted by adjusting the currents supplied to the two sets of lasers. Finer adjustment may be realized by providing a larger number of laser sets focused respectively on the corresponding number of line beams (e.g., four line beams), and programming the four current levels supplied to the four laser sets. 
     DSA Apparatus: 
     One embodiment of the DSA apparatus described in the above-referenced patent application is illustrated in the schematic orthographic representation of  FIG. 1 . A gantry structure  10  for two-dimensional scanning includes a pair of fixed parallel rails  12 ,  14 . Two parallel gantry beams  16 ,  18  are fixed together a set distance apart and supported on the fixed rails  12 ,  14  and are controlled by an unillustrated motor and drive mechanism to slide on rollers or ball bearings together along the fixed rails  12 ,  14 . A beam source  20  is slidably supported on the gantry beams  16 ,  18 , and may be suspended below the beams  16 ,  18  which are controlled by unillustrated motors and drive mechanisms to slide along them. A silicon wafer  22  or other substrate is stationarily supported below the gantry structure  10 . The beam source  20  includes a laser light source and optics (described below) to produce two closely spaced downwardly directed fan-shaped beams  24  and  25  that strike the wafer  22  as leading and trailing line beams  26  and  27 , respectively, extending generally parallel to the fixed rails  12 ,  14 , in what is conveniently called the slow direction. As will be described below, the second line beam  27  is parallel to the first line beam  26  and displaced from it by a distance corresponding to the minimum resolvable spot size of the optical system. How that is accomplished is discussed later in this specification. 
     Although not illustrated here, the gantry structure further includes a Z-axis stage for moving the laser light source and optics in a direction generally parallel to the fan-shaped beam  24  to thereby controllably vary the distance between the beam source  20  and the wafer  22  and thus control the focusing of the line beam  26  on the wafer  22 . Exemplary dimensions of the line beam  26  include a length of 1 cm and a width of 66 microns with an exemplary power density of 220 kW/cm 2 . Alternatively, the beam source and associated optics may be stationary while the wafer is supported on a stage which scans it in two dimensions. 
     In typical operation, the gantry beams  16 ,  18  are set at a particular position along the fixed rails  12 ,  14  and the beam source  20  is moved at a uniform speed along the gantry beams  16 ,  18  to scan the line beams  26 ,  27  perpendicularly to its long dimension in a direction conveniently called the fast direction. The two line beams  26 ,  27  are thereby scanned from one side of the wafer  22  to the other to irradiate a 1 cm swath of the wafer  22 . The line beams  26 ,  27  are sufficiently narrow and the scanning speed in the fast direction is sufficiently fast so that a particular area of the wafer is only momentarily exposed to the optical radiation of the line beams  26 ,  27  but the intensity at the peak of the line beam is sufficient to heat the surface region to very high temperatures. However, the deeper portions of the wafer  22  are not significantly heated and further act as a heat sink to quickly cool the surface region. Once the fast scan has been completed, the gantry beams  16 ,  18  are moved along the fixed rails  12 ,  14  to a new position such that the line beam  26  is moved along its long dimension extending along the slow axis. The fast scanning is then performed to irradiate a neighboring swath of the wafer  22 . The alternating fast and slow scanning are repeated, perhaps in a serpentine path of the beam source  20 , until the entire wafer  22  has been thermally processed. 
     The optics beam source  20  includes an array of lasers in order to realize the high optical power density (220 kW/cm 2 ) required. The optical system described below focuses the beams from the array of lasers into two closely spaced parallel line beams, each of width 66 microns. One optical system that is suitable for doing this is orthographically illustrated in  FIGS. 2 and 3 , in which laser radiation at about 810 nm is produced in an optical system  30  from two laser bar stacks  32 , one of which is illustrated in end plan view in  FIG. 4 . Each laser bar stack  32  includes 14 parallel bars  34 , generally corresponding to a vertical p-n junction in a GaAs semiconductor structure, extending laterally about 1 cm and separated by about 0.9 mm. Typically, water cooling layers are disposed between the bars  34 . In each bar  34  are formed  49  emitters  36 , each constituting a separate GaAs laser emitting respective beams having different divergence angles in orthogonal directions. The illustrated bars  34  are positioned with their long dimension extending over multiple emitters  36  and aligned along the slow axis and their short dimension corresponding to the less than 1-micron p-n depletion layer aligned along the fast axis. The small source size along the fast axis allows effective collimation along the fast axis. The divergence angle is large along the fast axis and relatively small along the slow axis. 
     Returning to  FIGS. 2 and 3 , individual half-cylindrical lenslets  40  are positioned over individual laser bars  34  to collimate the laser light in a narrow beam along the fast axis. The lensets  40  may be bonded with adhesive on the laser stacks  32  and aligned with the bars  34  to extend over the emitting areas  36 . As will be described in more detail later in this specification, some of the laser bars  34  and their lenlets  40  are covered by prisms  44  that deflect the light by an angle less than 1 degree to form the second (trailing) line beam  27  depicted in  FIG. 1 . The angle of the prisms  44  and the displacement between the two line beams  26 ,  27  are so infinitesimally small relative to the scale of the drawings of  FIGS. 1-4  that their size in the drawings has been greatly exaggerated in order for them to be slightly visible in the drawings. 
     The optics beam source  20  can further include conventional optical elements. Such conventional optical elements can include an interleaver and a polarization multiplexer, although the selection by the skilled worker of such elements is not limited to such an example. In the example of  FIGS. 2 and 3 , the two sets of beams from the two bar stacks  32  are input to an interleaver  42 , which has a multiple beam splitter type of structure and having specified coatings on two internal diagonal faces, e.g., reflective parallel bands, to selectively reflect and transmit light. Such interleavers are commercially available from Research Electro Optics (REO). In the interleaver  42 , patterned metallic reflector bands are formed in angled surfaces for each set of beams from the two bar stacks  32  such that beams from bars  34  on one side of the stack  32  are alternatively reflected or transmitted and thereby interleaved with beams from bars  34  on the other side of the stack  32  which undergo corresponding selective transmission/reflection, thereby filling in the otherwise spaced radiation profile from the separated emitters  36 . 
     A first set of interleaved beams is passed through a half-wave plate  48  to rotate its polarization relative to that of the second set of interleaved beams. Both sets of interleaved beams are input to a polarization multiplexer (PMUX)  52  having a structure of a double polarization beam splitter. Such a PMUX is commercially available from CVI Laser Inc. First and second diagonal interface layers  54 ,  56  cause the two sets of interleaved beams to be reflected along a common axis from their front faces. The first interface  54  is typically implemented as a dielectric interference filter designed as a hard reflector (HR) while the second interface  56  is implemented as a dielectric interference filter designed as a polarization beam splitter (PBS) at the laser wavelength. As a result, the first set of interleaved beams reflected from the first interface layer  54  strikes the back of the second interface layer  56 . Because of the polarization rotation introduced by the half-wave plate  48 , the first set of interleaved beams passes through the second interface layer  56 . The intensity of a source beam  58  output by the PMUX  52  is doubled from that of the either of the two sets of interleaved beams. 
     Although shown separated in the drawings, the interleaver  42 , the half-wave plate  48 , and the PMUX  52  and its interfaces  54 ,  56 , as well as additional filters that may be attached to input and output faces are typically joined together by a plastic encapsulant, such as a UV curable epoxy, to provide a rigid optical system. There is a plastic bond between the lenslets  40  and the laser stacks  32 , on which they are aligned to the bars  34 . The source beam  58  is passed through a set of cylindrical lenses  62 ,  64 ,  66  to focus the source beam  58  along the slow axis. 
     A one-dimensional light pipe  70  homogenizes the source beam along the slow axis. The source beam, focused by the cylindrical lenses  62 ,  64 ,  66 , enters the light pipe  70  with a finite convergence angle along the slow axis but substantially collimated along the fast axis. The light pipe  70 , more clearly illustrated in the orthographic view of  FIG. 5 , acts as a beam homogenizer to reduce the beam structure along the slow axis introduced by the multiple emitters  36  in the bar stack  32  spaced apart on the slow axis. The light pipe  70  may be implemented as a rectangular slab  72  of optical glass having a sufficiently high index of refraction to produce total internal reflection. It has a short dimension along the slow axis and a longer dimension along the fast axis. The slab  72  extends a substantial distance along an axis  74  of the source beam  58  converging along the slow axis on an input face  76 . The source beam  58  is internally reflected several times from the top and bottom surfaces of the slab  72 , thereby removing much of the texturing along the slow axis and homogenizing the beam along the slow axis when it exits on an output face  78 . The source beam  58 , however, is already well collimated along the fast axis (by the cylindrical lenslets  40 ) and the slab  72  is wide enough that the source beam  58  is not internally reflected on the side surfaces of the slab  72  but maintains its collimation along the fast axis. 
     The light pipe  70  may be tapered along its axial direction to control the entrance and exit apertures and beam convergence and divergence. The one-dimensional light pipe can alternatively be implemented as two parallel reflective surfaces corresponding generally to the upper and lower faces of the slab  72  with the source beam passing between them. The source beam output by the light pipe  70  is generally uniform. 
     As further illustrated in the schematic view of  FIG. 6 , further anamorphic lens set or optics  80 ,  82  expands the output beam in the slow axis and includes a generally spherical lens to project the desired line beam  26  on the wafer  22 . The anamorphic optics  80 ,  82  shape the source beam in two dimensions to produce a narrow line beam of limited length. In the direction of the fast axis, the output optics have an infinite conjugate for the source at the output of the light pipe (although systems may be designed with a finite source conjugate) and a finite conjugate at the image plane of the wafer  22  while, in the direction of the slow axis, the output optics has a finite conjugate at the source at the output of the light pipe  70  and a finite conjugate at the image plane. Further, in the direction of the slow axis, the nonuniform radiation from the multiple laser diodes of the laser bars is homogenized by the light pipe  70 . The ability of the light pipe  70  to homogenize strongly depends on the number of times the light is reflected traversing the light pipe  70 . This number is determined by the length of the light pipe  70 , the direction of the taper if any, the size of the entrance and exit apertures as well as the launch angle into the light pipe  70 . Further anamorphic optics focus the source beam into the line beam of desired dimensions on the surface of the wafer  22 . 
     Programmable Beam Profile for Dwell Time and Temperature Profile Control: 
     Referring to  FIGS. 6 and 8 , one group of laser bars  34  (e.g., alternate bars  34 ) radiate through corresponding cylindrical lenslets  40  to the optics  42 ,  52 , etc. The remaining ones of the lasers bars  34  radiate through corresponding cylindrical lenslets  40  and through individual prisms  44  overlying respective ones of the lenslets  40 . Each prism  44  deflects the beam by an angle (A) less than 1 degree through an angle of rotation about an axis parallel with the fast axis to produce a beam deflection perpendicular to the fast axis. The first group of laser bars  34  whose beams are undeflected generate the leading beam  24 . The second group of laser bars  34  whose beams are deflected by the respective prisms  44  generate the trailing beam  25 . The perspective view of  FIG. 7  shows two of the laser bars  34  forming two parallel rows of emitters  36 , both covered by respective cylindrical lenslets  40 , and one of the lenslets  40  being covered by a prism  44 .  FIG. 8  illustrates how the power densities of the leading and trailing beams  24 ,  25  are independently governed by a profile controller  100  controlling current supplies  102 ,  104  that drive the emitters  36  of alternate laser bars  34 . The first current supply  102  is the front beam current supply because it is coupled to each of the laser bars not covered by any of the prisms  40 . The second current supply  104  is the trailing beam current supply because it is coupled to each of the laser bars covered by a prism  44 . Each prism  44  deflects the corresponding laser beam to form the trailing beam  25 . The two line beams  26 ,  27  ( FIG. 1 ) imaged on the wafer  22  by the leading and trailing beams  24 ,  25  are separated by a displacement determined by the deflection angle of the prisms  44 . The terms “front” and “trailing” apply to an embodiment in which the beam is scanned across the wafer in a particular direction. If this direction is reversed, then the beams deflected by the prisms  44  form the leading or “front” beam  26  while the undeflected beams form the trailing beam  27 . The invention may be carried out in either mode, and therefore the terms such as “front” and “trailing”, for example, are employed interchangeably with respect to the embodiment of  FIG. 6 . 
     In operation, the power density of the trailing line beam  27  may be significantly less than that of the front line beam  26 . This is because the front line beam  26  must have sufficient power density to raise the temperature of the wafer surface rapidly through the lower temperature ranges where the higher thermal conductivity of the wafer makes it more difficult to heat the surface, until the wafer surface reaches the peak temperature. The trailing line beam merely maintains this temperature, which requires less power density, to avoid raising the wafer surface temperature beyond the maximum desired temperature (1300 degrees C). Therefore, as indicated in  FIG. 9 , the trailing line beam has a fast axis profile whose peak power density is significantly lower than that of the leading line beam. The difference between the peak power density levels of the two beams is set by the profile controller  100  of  FIG. 8 . 
       FIG. 10  depicts an embodiment providing several (i.e., four) trailing beams in succession, each one with an independently adjustable power density. In  FIG. 10 , the laser bars  34  are divided into four groups. The one group of laser bars  34 - 0  produce laser radiation that is undeflected, to form the front beam. Another group of laser bars  34 - 1  produce radiation that is deflected by prisms  44 - 1  through a small angle A 1  to produce the first trailing beam. A further group of laser bars  34 - 2  produce radiation that is deflected by prisms  44 - 2  through an angle A 2  that is greater than A 1 . Another group of laser bars  34 - 3  produce radiation that is deflected by prisms  44 - 3  through an angle A 3  that is greater than A 2 . The four line beams  26 - 1 ,  26 - 2 ,  26 - 3 ,  26 - 4  ( FIG. 11A ) are focused by the optics ( 42 ,  52 , etc. of  FIGS. 2-6 ) on the wafer surface. In the special case in which the four line beams  26 - 1 ,  26 - 2 ,  26 - 3 ,  26 - 4  have the same power density, their power density profiles along the fast axis are of identical Gaussian shapes, but are shifted from one another along the fast axis by the same peak-to-peak displacement, as depicted in  FIG. 11A . This displacement is determined by the succession of deflection angles A 1 , A 2 , A 3  imposed by the prisms  44 - 1 ,  44 - 2 ,  44 - 3 . Preferably, the angles A 1 , A 2 , A 3  are selected so that the peak-to-peak displacement between neighboring line beams is at least approximately (if not exactly) equal to the half-maximum beam width (depicted in  FIG. 9 ) of a single line beam (e.g., of the front beam). Equivalently, the displacement may correspond to the minimum resolvable spot size of the beam, discussed previously in this specification. The deflection angles are all less than 1 degree and in reality would not be detectable in the drawing of  FIG. 10 . These angles have been exaggerated in  FIG. 10  for the sake of illustration. 
     The power density levels produced by the different groups of laser bars  34 - 0 ,  34 - 1 ,  34 - 2 ,  34 - 3  are independently adjustable by a laser power controller  110  that furnishes independent supply currents I 0 , I 1 , I 2 , I 3  to the respective laser bars  34 - 0 ,  34 - 1 ,  34 - 2 ,  34 - 3 . The laser power controller  110  therefore controls the fast axis power density profile produced by the array of lasers. While their power densities may be the same (corresponding to the power density profile of  FIG. 11A ), it is preferable for the power densities of the trailing line beams to be less than that of the front or leading line beam, in accordance with the various power density profiles of  FIGS. 11B ,  11 C or  11 D. The successive line beam supply current levels I 0 , I 1 , I 2 , I 3  may be adjusted by the controller  110  to produce a staircase profile of successively decreasing power density levels as in  FIG. 11B , or the single staircase of  FIG. 11C , or the gradual staircase of  FIG. 11D . As another alternative, the amplitude profile may fall after the front or leading line beam and then increase from the third to last line beam, as depicted in  FIG. 11E . Although not depicted in the drawings, the profile may be further adjusted to produce an ascending staircase pattern instead of the descending staircase profiles of  FIGS. 11B-11D . 
     The presently preferred power density profile of the succession of line beams is illustrated in FIG.  12 A.  FIG. 12A  depicts the power density incident upon a particular spot on the wafer surface as a function of time. (If  FIG. 12A  were converted to depict power density distribution along the fast axis at a fixed instant in time, then the graph would remain unchanged, depending upon the units chosen.) The leading line beam  26 - 1  has the highest peak power density, while the successive trailing line beams  26 - 2 ,  26 - 3 ,  26 - 4  have a lesser peak power density, which is the same for each of them. The strong leading beam  26 - 1  of  FIG. 12A  has sufficient power density to overcome the high silicon thermal conductivity to rapidly heat the wafer surface spot to 1300 degrees C. The trailing beams  26 - 2 ,  26 - 3 ,  26 - 4  that follow have a lesser power density that is sufficient only to maintain the 1300 degree C wafer temperature at the spot and not exceed it.  FIG. 12B  depicts the temperature behavior over time of the same wafer surface spot that results from the beam profile of  FIG. 12A . The temperature rises rapidly from 400 degrees C to 1300 degrees C with the leading edge of the front beam. The temperature of the spot then remains at about 1300 degrees C for 3 ms, with slight undulations in the temperature corresponding with the peaks of the successive beams. After about 3 ms, the temperature falls rapidly to 400 degrees C with the trailing edge of the last line beam. 
       FIG. 12B  indicates the succession of effects of the laser radiation in a post-ion implantation DSA annealing process using the beam profile of  FIG. 12A . During an initial time interval (T 1 ), ion bombardment damage in the semiconductor material incurred during a pre-implant amorphization process is annealed to convert the semiconductor material from a partially amorphous state to a crystalline state. During a further time interval, T 2 , the implanted dopant impurities are rendered substitutional in the semiconductor crystal lattice. During a yet further time interval T 3 , boundary defects formed at the interface between the bulk crystal and the re-crystallized zone are cured.  FIG. 13  illustrates the location of such boundary defects. The boundary defects arise when the amorphized semiconductor surface layer is re-crystallized to form an epitaxial crystal layer over the bulk crystal. The two crystal zones may not align perfectly at the boundary between them, giving rise to misalignments that are defects. Such defects may require as long as 3 ms at 1300 degrees C. (i.e., the entire duration of the multiple line beams of  FIG. 12A ) to completely anneal or cure. 
       FIG. 14  depicts an ion implantation process for forming ultra-shallow PN junctions such as source-drain extension implants in the surface of a silicon wafer. The structure to be formed is depicted in  FIG. 13 , in which ion implanted source-drain extensions  200 ,  205  are implanted between deep source-drain contact regions  210 ,  215  and a semiconductor channel region  220  underlying a gate electrode  225  insulated from the channel region  220  by a thin gate dielectric layer  230 . In the first step of the process of  FIG. 14  (block  250 ), the surface region extending from the top surface  235  of the wafer to the horizontal dashed line  240  is converted from a pure crystalline state to an at least partially amorphous state by ion bombarding the wafer with heavy ions (e.g., oxygen, nitrogen, carbon, germanium) to break the crystal bonds. This ion bombardment is carried out at an energy level at which the distribution of ions in the wafer extends down to the dashed line  240  and is virtually cut-off below that line. The purpose of this amorphization step is to prevent channeling of dopant impurity ions that are to be implanted in the next step of the process. Channeling is made possible by the regular crystalline structure of the silicon wafer, and is prevented by converting the crystalline structure to an amorphous one to a sufficient depth (corresponding to the dashed line  240  of  FIG. 13 ). 
     The next step (block  255  of  FIG. 14 ) is to implant a dopant impurity to form, for example, the source drain extensions  210 ,  215 . The dopant impurity may be As, P, B or other species. This step may be preceded by masking steps to shield areas of the wafer that are not to be implanted in this step. The ion energy is selected so that the implanted ion distribution does not extend below the desired depth, such as the ultra-shallow depth of the source drain extensions  210 ,  215  for example. 
     Optionally, prior to performing a DSA process employing the apparatus of  FIGS. 1-10 , an optical absorber layer may be deposited on the wafer surface (block  260  of  FIG. 14 ). This step may be carried out in accordance with the low-temperature plasma process and apparatus described in U.S. patent application Ser. No. 11/131,904, filed May 17, 2005, entitled A SEMICONDUCTOR JUNCTION FORMATION PROCESS INCLUDING LOW TEMPERATURE PLASMA DEPOSITION OF AN OPTICAL ABSORPTION LAYER ND HIGH SPEED OPTICAL ANNEALING by Kartik Ramaswamy, et al. and assigned to the present assignee. The optical absorber layer may be amorphous carbon, for example. 
     The next step (block  270 ) is to perform the scanning laser DSA process using multiple line beams with a configurable beam profile. A first sub-step (block  271 ) of this step is to rapidly raise the temperature of a newly encountered wafer surface spot (or line of spots) from an ambient temperature of 400-450 degrees C up to 1300 degrees C using the steep leading edge of the front beam  26 - 1  ( FIG. 12A ). The next sub-step (block  272 ) is to maintain the wafer temperature at about 1300 degrees C for a sufficient time (e.g., 3 ms) to (a) re-crystallize the amorphized surface region, (b) render the implanted dopant impurities substitutional in the re-crystallized lattice and (c) cure the defects at the boundary between the re-crystallized zone and the underlying bulk crystal. The final sub-step (block  273 ) is to rapidly reduce the spot surface temperature in accordance using the steep trailing edge of the last trailing beam  26 - 4  ( FIG. 12A ). 
       FIG. 15  depicts a modification of the embodiment of  FIG. 8  in which the prisms  44  are replaced by separate mirrors  121 ,  122 ,  123  that deflect the beams from alternate laser bars  34  in the same manner that the prisms  44  deflected the beams in the embodiment of  FIG. 8 . If the beam deflection angle is A, then the angle of each mirror  121 ,  122 ,  123  relative to the beam direction emerging from each laser bar  34  is A/2. 
       FIG. 16  depicts a simpler version of the embodiment of  FIG. 15 , in which a single optical element  120 , which may be a mirror or a prism, deflects the beams from a succession of lasers bars  34 - 1 ,  34 - 2 ,  34 - 3 , etc., to produce the trailing line beam, while the beams from the other laser bars  34 - 4 ,  34 - 5 ,  34 - 6  are undeflected to form the leading line beam. In the case in which the optical element  120  of  FIG. 16  is a mirror rather than a prism, for a beam deflection angle A, the angle of the optical element  120  relative to the beam direction emerging from each laser bar  34  is A/2, as in the embodiment of  FIG. 15 . 
       FIGS. 17 and 18  depict a modification of the embodiment of  FIGS. 7 and 8 , in which the prisms  44  are eliminated and, instead, different (i.e., alternate) ones of the half-cylindrical lenses  40  are rotated through the angle A. Rotating the selected cylindrical lenses  40  produces the same beam deflection as did the prisms  44  of  FIGS. 7 and 8 . The half-cylindrical lenses  40  are mounted on the respective laser bars  34  and aligned to produce the desired beam direction and then bonded to the laser bars, preferably with UV curable epoxy. Thus, half the lenses ( 40 - 1 ,  40 - 3 ,  40 - 5 ) are aligned to provide a beam direction deflected by the angle A, while the remaining lenses  40 - 2 ,  40 - 4 ,  40 - 6  are aligned to provide an undeflected beam direction. 
       FIG. 19  depicts a modification of the embodiment of  FIG. 10  in which the prisms  44 - 1 ,  44 - 2 ,  44 - 3  of successively greater beam angles are eliminated and their beam deflection functions are provided instead by rotating corresponding ones of the half-cylindrical lenses ( 40 - 1 ,  40 - 2 ,  40 - 3 ) through successively greater angles A 1 , A 2 , A 3 . The lens  40 - 0  remains unrotated, to provide four successive beam angles of  0 , A 1 , A 2  and A 3 . This is identical to the succession of beam angles provided to the optics  42 ,  52 , in the embodiment of  FIG. 10 . 
       FIG. 20  depicts a modification of the embodiment of  FIG. 10  in which the prisms  44 - 1 ,  44 - 2 ,  44 - 3  of successively greater beam angles are replaced by mirrors  120 - 1 ,  120 - 2 ,  120 - 3  rotated by successively greater angles A 1 /2, A 2 /2, A 3 /2 deflecting the beams from the laser bars  34 - 1 ,  34 - 2 ,  34 - 3 , respectively. The beams from the laser bar  34 - 0  is undeflected, to provide four successive beam angles of  0 , A 1 , A 2  and A 3 . This is identical to the succession of beam angles provided to the optics  42 ,  52 , in the embodiment of  FIG. 10 . 
     While the invention has been described in detail with reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.