Patent Publication Number: US-2009236495-A1

Title: Autofocus for high power laser diode based annealing system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/198,660, filed Aug. 5, 2005 entitled AUTOFOCUS FOR HIGH POWER LASER DIODE BASED ANNEALING SYSTEM by Dean Jennings, et al., which claims the benefit of U.S. Provisional Application No. 60/627,530, filed Nov. 12, 2004. 
    
    
     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 
     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° C. to greater than 1000°, 1200°, or even 1400° 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 achieve the desired processing temperature. The time that the wafer needs to stay at the highest temperature can be very short. 
     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 by Jennings et al. in PCT/2003/00196966 based upon U.S. patent application Ser. No. 10/325,497, filed Dec. 18, 2002 and incorporated herein by reference in its entirety. Markle describes a different form in U.S. Pat. No. 6,531,681 and Talwar yet a further version in U.S. Pat. No. 6,747,245. 
     The Jennings and Markle versions use CW diode lasers to produce very intense beams of light that strikes the wafer as a thin long line of radiation. The line is then scanned over the surface of the wafer in a direction perpendicular to the long dimension of the line beam. 
     SUMMARY 
     Apparatus for thermally processing a substrate includes a source of laser radiation comprising a plurality diode lasers arranged along a slow axis, optics directing the laser radiation from the source to the substrate, and an array of photodetectors arranged along a fast axis perpendicular to the slow axis and receiving portions of the laser radiation reflected from the substrate through the optics. The diode lasers may be implemented as parallel rows of diode lasers along the slow axis. The apparatus can further include a first translation mechanism for varying a distance between (a) the substrate and (b) the optics, and a controller receiving an output of the array of photodetectors and controlling the translation mechanism in response to the output to focus the laser radiation on the substrate. Preferably, the optics are configured to focus the laser radiation on the substrate in a line beam having a long dimension along the slow axis and a short dimension along the fast axis. A second translation mechanism moves the optics and the substrate relative to each other at least in the fast direction. 
    
    
     
       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 of a preferred embodiment. 
         FIG. 7  is an orthographic projection representative of the embodiment of  FIG. 6 . 
         FIGS. 8 ,  9  and  10  are diagrams of ray traces in the apparatus of  FIG. 7  corresponding optics-to-substrate distances that are in-focus, too close and too far, respectively. 
         FIGS. 11 ,  12  and  13  are graphic representations of the incident and reflected light beams along the fast axis for optics-to-substrate distances that are in-focus, too close and too far, respectively. 
         FIG. 14  is a graph of nominal best focus fast axis light intensity profiles for two different optics-to-substrate distances. 
     
    
    
     DETAILED DESCRIPTION 
     One embodiment of the apparatus described in the above-referenced application by Jennings et al. 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 to produce a downwardly directed fan-shaped beam  24  that strikes the wafer  22  as a line beam  26  extending generally parallel to the fixed rails  12 ,  14 , in what is conveniently called the slow direction. 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 beam  26  perpendicularly to its long dimension in a direction conveniently called the fast direction. The line beam  26  is thereby scanned from one side of the wafer  22  to the other to irradiate a 1 cm swath of the wafer  22 . The line beam  26  is narrow enough and the scanning speed in the fast direction fast enough that a particular area of the wafer is only momentarily exposed to the optical radiation of the line beam  26  but the intensity at the peak of the line beam is enough 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. An example 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  two arrays of cylindrical lenslets  40  are positioned along the laser bars  34  to collimate the laser light in a narrow beam along the fast axis. They may be bonded with adhesive on the laser stacks  32  and aligned with the bars  34  to extend over the emitting areas  36 . 
     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 quarter-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 Research Electro Optics. 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 quarter-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 quarter-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. An important interface is the plastic bonding of the lenslets  40  to the laser stacks  32 , on which they must be 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 . It is desired to provide autofocusing capability such that the distance between the optics and the wafer  22  may be automatically adjusted in real time to provide a well focused line beam on the wafer  22 . Autofocusing is well known in cameras. However, conventional autofocusing techniques cannot easily be applied to the complex optics shown in  FIGS. 2 and 3 . In particular, the single-axis light pipe  70  providing beam homogenization along the slow axis is inconsistent with conventional autofocusing techniques and mechanisms. 
     In one aspect of the invention, the reflections of the line beam  26  from the wafer are monitored for variations along the fast axis only. As illustrated in  FIG. 6  and the orthographic view of  FIG. 7 , the same optics  62 ,  64 ,  66 ,  80  that project the line beam  26  onto the wafer  22  are also used to guide laser light reflected from the wafer  22  to a CCD array  90  positioned on a side of the PMUX  52  opposite the wafer  22 . The light pipe  70  is intermediate the optics  80  and the optical lenses  62 ,  64 ,  66 . The CCD array  90  includes a plurality of photosensitive charge coupled devices arranged along one axis and possibly along two axes. However, other types of linear or rectangular photodetectors may be employed. The wavelength-selective reflective surfaces  54 ,  56  in the PMUX  52  are sufficiently inefficient (i.e., have a coefficient of reflection that is less that 100%) so that a small but finite fraction of the laser light reflected from the wafer  22  passes through them to a cylindrical lens  92  ( FIG. 6 ) focusing the reflected light along the slow axis but leaving it substantially collimated along the fast axis as it strikes the CCD array  90 . (The fast and slow axes may be projected onto both the wafer  22  as well as the CCD array  90 .) The cylindrical lens  92  may be used to reduce the aperture in the slow direction to increase optical throughput since the light pipe  70  suppresses information in this direction. The CCD array  90  provides a fast-axis profile of the light to a source controller  94 , which determines if the line beam  26  is adequately focused on the wafer  22  or whether the separation between the beam source  20  and the wafer is too large or too small. Accordingly, the source controller  94  causes a Z-axis stage  96  to which the beam source  20  is mounted in the gantry structure of  FIG. 1  to move the beam source  20  toward or away from the wafer for a better focus. It is of course appreciated that the wafer  22  could be supported on a Z-axis stage so it could be moved toward or away from the beam source  20 , which otherwise is stationary along the Z-axis. 
     The angular distribution of light along the fast axis contains information about the location of the reflecting plane, that is, the wafer  22 . A model of the ideal (in-focus) fast axis light distribution or profile at the CCD array  90  is determined as are expected fast axis distributions for out-of-focus positions. The return or reflected light distribution is fit to the best model and the focus condition is thereby determined. The nominal best focus is the condition in which all light returns to the same aperture size along the fast axis (at the CCD array  90 ) as the source aperture for the light initially directed from the laser bars  32  to the wafer. 
       FIG. 8  illustrates the nominal best focus along the fast axis in which the optics  62 ,  64 ,  66 ,  80  focus substantially collimated laser radiation  100  to the line beam on the wafer  22 . The outline further shows an incident path  102  and reflected path  104  for a ray on the periphery of the beam arranged around a central axis  106 . However, if the wafer  22  is too close to the optics, as illustrated in  FIG. 9 , the reflected path  104   b  is closer to the central axis  106  and produces a narrower collimated return beam. On the other hand, if the wafer  22  is too far from the optics, as illustrated in  FIG. 10 , the return path  104   c  is further away from the central axis  106  and produces a wider collimated return beam. It is understood that incident rays need to be traced from each side of the central axis  106  to provide a fast-axis profile. 
     A more complete simulation shows the collimated source beam  110  and the collimated return beam  112  near the PMUX for the condition of nominal best focus in  FIG. 11 , the condition of the wafer being too close to the source in  FIG. 12 , and the condition of the wafer being too far from the source in  FIG. 13 . Line  114  in the graph of  FIG. 14  shows the fast-axis intensity profile for nominal best focus and line  116  shows the condition of the wafer being 500 microns too far from the source. 
     It is understood that in some more fundamental aspects of the invention not requiring scanning a line beam, the fast and slow axes can be understood simply as perpendicular first and second axes. It is also understood that the fast and slow axes may change in absolute space as optics redirect the principal optical axis. 
     In focusing the laser beam, it may not be necessary in some alternative embodiments to move the lasers  32  relative to the substrate  22  or even include the lasers  32  within the optics source  20 , provided the other components of the optics source  20  (e.g., lenses  62 ,  64 ,  66 ,  80 , and homogenizer  70 ) are moved relative to the substrate  22 . 
     It may be possible to carry out the invention without either the interleaver  42  or the polarization multiplexer  52  or without both of them. As one possible example, an optical element similar to the reflective surfaces  54 ,  56  (or similar to one of them) may be employed to carry out the invention, that element not being part of a polarization multiplexer. While the invention has been described in detail by specific 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.