Abstract:
Apparatus for thermally processing a semiconductor wafer includes an array of semiconductor laser emitters arranged in plural parallel rows extending along a slow axis, plural respective cylindrical lenses overlying respective ones of the rows of laser emitters for collimating light from the respective rows along a fast axis generally perpendicular to the slow axis, a homogenizing light pipe having an input face at a first end for receiving light from the plural cylindrical lenses and an output face at an opposite end, the light pipe comprising a pair of reflective walls extending between the input and output faces and separated from one another along the direction of the slow axis, and scanning apparatus for scanning light emitted from the homogenizing light pipe across the wafer in a scanning direction parallel to the fast axis.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/627,238, filed Nov. 12, 2004. 
   This application contains subject matter related to U.S. application Ser. No.: 11/185,454 filed Jul. 20, 2005 entitled RAPID DETECTION OF IMMINENT FAILURE IN LASER THERMAL PROCESSING OF A SUBSTRATE by Bruce Adams, et al.; U.S. patent application Ser. No.: 11/185,651 filed Jul. 20, 2005 entitled THERMAL FLUX LASER ANNEALING FOR ION IMPLANTATION OF SEMICONDUCTOR P-N JUNCTIONS by Bruce Adams, et al.; U.S. application Ser. No.: 11/195,380 filed Aug. 2, 2005 entitled MULTIPLE BAND PASS FILTERING FOR PYROMETRY IN LASER BASED ANNEALING SYSTEMS by Bruce Adams, et al.; and U.S. 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., all of which applications are assigned to the present assignee. 

   BACKGROUND OF THE INVENTION 
   1. 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. 
   2. 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° 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 OF THE INVENTION 
   Apparatus for thermally processing a semiconductor wafer includes an array of semiconductor laser emitters arranged in plural parallel rows extending along a slow axis, plural respective cylindrical lenses overlying respective ones of the rows of laser emitters for collimating light from the respective rows along a fast axis generally perpendicular to the slow axis, a homogenizing light pipe having an input face at a first end for receiving light from the plural cylindrical lenses and an output face at an opposite end, the light pipe comprising a pair of reflective walls extending between the input and output faces and separated from one another along the direction of the slow axis, and scanning apparatus for scanning light emitted from the homogenizing light pipe across the wafer in a scanning direction parallel to the fast axis. Lenses focus light derived from the output face of the light pipe into a line of light on the wafer, the line of light having an elongate dimension along the slow axis and a narrow dimension along the fast axis, wherein the scanning apparatus scans the line of light across the wafer along the fast axis. The reflective walls of the light pipe are sufficiently close to one another to facilitate multiple reflections across the slow axis. 

   
     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 perspective view of the light pipe of  FIG. 5  and of the lens assemblies at its input and output faces. 
       FIG. 7  is a top view of the light pipe of  FIG. 6  along the fast axis. 
       FIG. 8  is a side view of the light pipe of  FIG. 6  along the slow axis. 
       FIG. 9  is an orthographic view of an embodiment of the light pipe of  FIG. 5  formed as a truncated wedge having decreasing cross-sectional area along the optical axis. 
       FIG. 10  is an orthographic view of an embodiment of the light pipe of  FIG. 5  formed as a truncated wedge having increasing cross-sectional area along the optical axis. 
       FIG. 11  is diagram of multiple reflections inside the light pipe of  FIG. 10 , illustrating the effects of a beam diverging lens at the input of the light pipe. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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 lensets  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  that includes cylindrical lenses  81 ,  82 , expands the output beam in the slow axis, and further includes a generally spherical lens  83  to project the desired line beam  26  on the wafer  22 . The anamorphic optics  80  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 . The output optics  80  focus the source beam into the line beam of desired dimensions on the surface of the wafer  22 . 
     FIGS. 7 and 8  are perpendicularly arranged side views along the fast and slow axes respectively showing the light pipe  70  and some associated optics. In the direction of the fast axis, the beam from the lasers bars  32  is well collimated and not affected by the light pipe  70  or anamorphic optics. On the other hand, in the direction of the slow axis, the input anamorphic optics  62 ,  64 ,  66  condense and converge the beam into the input end of the light pipe  70 . The beam exits the light pipe  70  with substantially uniform intensity along the slow axis but with a substantial divergence. The output anamorphic optics  80  expand and collimate the output beam along the slow axis. 
   The light pipe  70  described above has a uniform rectangular cross section along the optical axis  74 . However, tapered profiles with cross sections tapering along the optical axis  74  may be advantageously used in combination with the subsequent optics. In particular, a tapered light pipe increases the number of reflections occurring over a fixed length of the light pipe. A dielectric light pipe  90  illustrated orthographically in  FIG. 9  is formed from a truncated wedge  92  of optical glass with a uniformly decreasing rectangular cross section along the optical axis  74  from an input face  94  to an output face  96 . That is, the aspect ratio of the light pipe  90  is continually increasing, for example, from 5:1 to 10:1, producing a ratio of aspect ratios, for example, of at least 2. In particular, the dimension along the slow axis is decreasing and the dimension along the fast axis may be maintained constant. The advantage of the narrow output face  96  is that its numerical aperture (NA) is higher, that is, the output beam divergence is greater. 
   A complementary configuration is a dielectric light pipe  100  illustrated orthographically in  FIG. 10  formed of a truncated wedge  102  of optical glass with a uniformly increasing rectangular cross section along the optical axis from an input face  104  to an output face  106  so that the aspect ratio of the wedge  102  is continually decreasing, for example, by ratios reverse to those of the previous embodiment. In particular, the dimension along the slow axis is increasing and the dimension along the fast axis may be maintained constant. This configuration has the advantage that the NA of the wide output face  106  is lower and the output beam divergence is less. Advantageously a cylindrical lens  108  placed near the input face and extending along the long lateral direction of the light pipe  100  focuses a somewhat collimated input beam  112  into a sharply converging beam at the input face  104 . As illustrated in the side cross sectional view of  FIG. 11 , lateral beam components  114  at the ends of the slow direction bounce many times near the small end of the tapered light pipe  100  and are gradually brought closer to be parallel to the optical axis. As a result, the output beam has a small NA and relatively large size along the slow axis. 
   It is appreciated that the lateral side walls of the dielectric light pipes  70 ,  90 ,  100  do not really participate in the action of the light pipe such that a single-axis light pipe is obtained in which no reflecting or homogenizing is obtained in along the long lateral direction of the pipe. Hence, it is not required that those laterals walls be parallel although such parallel walls ease fabrication. 
   The one-dimensional light pipe can alternatively be implemented as two parallel or slightly inclined reflective surfaces corresponding generally to the upper and lower faces of the slab  72  or wedges  92 ,  102  with the source beam passing between them. The reflective surfaces can be formed as free-standing mirrors or as coatings on a transparent member not providing total internal reflection. It may be possible to carry out the invention without either the interleaver  42  or the polarization multiplexer  52  or without both of them. 
   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.