Abstract:
A method utilizing spatially selective laser doping for irradiating predetermined portions of a substrate of a semiconductor material is disclosed. Dopants are deposited onto the surface of a substrate. A pulsed, visible beam is directed to and preferentially absorbed by the substrate only in those regions requiring doping. Spatial modes of the incoherent beam are overlapped and averaged, providing uniform irradiation requiring fewer laser shots. The beam is then focused to the predetermined locations of the substrate for implantation or activation of the dopants. The method provides for scanning and focusing of the beam across the substrate surface, and irradiation of multiple locations using a plurality of beams. The spatial selectivity, combined with visible laser wavelengths, provides greater efficiency in doping only desired substrate regions, while reducing the amount of irradiation required. Savings in cost and manufacturing throughput can be achieved, particularly with respect to doping poly-crystalline silicon.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of fabricating and manufacturing semiconductors. In particular the invention relates to the field of doping a substrate to form a semiconductor. Still further, this invention relates to the field of doping a semiconductor material in a predetermined pattern. Further, this invention relates to the field of spatially selective, laser assisted doping of a semiconductor substrate. 
     BACKGROUND OF THE INVENTION 
     This invention relates to the doping of a semiconductor material to provide a semiconductor device having a predetermined pattern of doped regions. 
     Additionally, this invention directs itself to a method of doping a semiconductor material where the doped regions are spatially selective. Additionally, this invention is directed to the doping of a semiconductor material or substrate with a pulsed visible laser beam which may be controlled to focus the laser beam on at least one selectively predetermined region of the substrate surface. 
     Still further, this invention is directed to a method of doping a semiconductor material whereby a spatially uniform beam is focused on a selectively predetermined region of the substrate surface for implanting dopants into the substrate. 
     This invention directs itself to a method of doping a semiconductor substrate without the necessity of having to mask a plurality of dopant regions on the surface of the substrate. 
     This invention relates to a method of doping a semiconductor substrate using a pulsed visible laser beam which can be controlled for focusing the laser beam on at least one of a plurality of doping regions on the substrate surface. 
     This invention is directed to a method of doping a semiconductor substrate while only irradiating specific regions of the semiconductor substrate wherein doping is required. 
     PRIOR ART 
     Traditional semiconductor manufacturing processes typically include ion implantation to introduce impurities/dopants into a substrate. Substrate temperatures sometimes in excess of 800° C. are required to activate the impurities. However, these high temperatures are generally not suitable for flexible electronic structures that contain a polymer substrate, or display devices utilizing low temperature processing of silicon (LTPS). 
     Laser assisted doping is a technique for activating the implanted ions or driving surface dopants into the substrate. In this process, a pulsed laser alternately melts and cools areas of a silicon substrate or other substrate material. Impurities delivered from a surrounding gas, for instance, can then diffuse into the molten regions. As the melted areas of the substrate cool (when the pulse of laser energy is not present), the impurities remain in the substrate. The resulting product may be a p-type or n-type semiconductor. This technique alternately melts and cools the substrate material, which correspondingly crystallizes the structure after cooling. 
     Current methods of manufacturing semiconductors include well known photo-lithographic processes such as mask, mask projection, or blanket scanning, used in conjunction with pulsed lasers.  FIG. 1  illustrates a conventional method of doping a semiconductor. Impurities or dopants  10  are deposited over the entire surface of a substrate  12 . A photo-lithographic mask  14  is then used to expose portions of the substrate to incoherent UV laser light  16 . The laser beam irradiates a portion of the substrate as defined by the pattern on the mask. The irradiated portions typically have a greater area than a particular element or feature requiring the doping, such as a TFT (thin film transistor). 
     This technique however suffers from various shortcomings: the emitted laser energy is an incoherent UV beam, and the pulsed light impinges on and illuminates a greater area of the substrate than is necessary. This is due to the fact that the TFT is confined to a small portion of the substrate, but the dopant covers the entire surface and the exposed portion is illuminated subject to the mask pattern. The end result is a highly inefficient process in which laser energy that is not needed is blocked by the mask. Additionally many laser shots are necessary due to the incoherent and unstable nature of the beam and its delivery to the surface of the material. 
     SUMMARY OF THE INVENTION 
     It is an objective of the invention to reduce the amount of laser energy delivered to the substrate in a doping process by selectively illuminating only those areas requiring impurities, without the use of a mask. 
     It is a further object to create various semiconductor materials in a lower temperature environment than is traditionally available. 
     It is a further object of the subject concept to dope a substrate through the use of a pulsed, visible laser beam. 
     An additional objective is to control a pulsed, visible laser beam to irradiate predetermined selective regions of a surface of the substrate. 
     It is yet another object to homogenize a multiplicity of spatial modes of the laser beam to produce a spatially uniform laser beam. 
     Another object of the subject concept is to focus the resulting homogenized beam onto only selectively predetermined regions of the substrate in order to implant the dopants in the substrate material. 
     A source of dopants is first deposited onto the entire surface of a substrate. A plurality of regions is then defined for subsequent irradiation. A focused pulsed, visible laser beam selectively irradiates only those regions on the substrate requiring doping, where TFTs (thin film transistors) are needed for example. The output of the laser is controlled in order to focus the beam onto these particularly chosen regions. Homogenization optics transforms the beam, which is highly incoherent, into a substantially spatially uniform beam by overlapping and averaging the spatial modes of the laser. This uniformity allows a greater concentration of the laser energy into useful spatial modes, reducing the number of laser shots needed to complete the doping by a factor of about 10 to 20 at each spatially selected region. The resulting beam is then focused onto the substrate by spatially selecting those areas requiring the dopant to be implanted in the substrate material. The choice of visible wavelengths dictates that the energy per photon will be smaller than typical UV wavelengths, enabling preferential absorption by materials such as poly-crystalline silicon and thereby reducing heat build-up on the surface. Areas of the substrate not requiring doping are not irradiated, thus saving laser energy and further avoiding excess heat deposition. 
     Additional aspects of the method allow for multiple laser beams to irradiate a plurality of predetermined regions of the substrate. This can be achieved by utilizing arrangements of light pipes or beam scanners such as 2-axis galvanometers, and even 3-axis stages. The latter element permits selective positioning of different doping regions of the substrate to be at the focus of the beam(s). The combination of visible wavelengths and spatially selective doping enables an overall savings in time and cost of manufacturing, while providing for greater efficiency and throughput. In the subject method a fraction of the substrate is doped and no mask is required, which relates to an economically advantageous method with less energy being expended than that found in prior art methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional semiconductor substrate with surface-deposited dopants, mask, and laser irradiation. 
         FIG. 2  is a schematic diagram of a semiconductor substrate with surface-deposited dopants and spatially selected laser irradiation, in accordance with the present invention concept. 
         FIG. 3  is a schematic diagram according to the invention showing homogenizing optics, aperture, and focusing of the laser beam onto spatially selected regions of the substrate. 
         FIG. 4  is a schematic diagram of an embodiment of the present invention using a 2-axis galvanometer to scan across the substrate, creating the effect of multiple laser beams irradiating multiple regions of the substrate. 
         FIG. 5  is a schematic diagram according to an embodiment of the present invention, illustrating relative movement between the substrate and the laser in order to move or spatially select another region for irradiation. 
         FIG. 6  is a schematic diagram of an embodiment of the present invention in which light pipes are arranged for irradiating a plurality of regions of the substrate. 
         FIG. 7  is a schematic diagram of another embodiment of the present invention in which the plurality of beams are provided by a holographic optical element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 2 , there is shown laser  18  which in general may be a visible laser such as an argon-ion, copper vapor, or frequency-converted Nd:YAG diode laser, which is displaced from substrate  12 . However, as will be explained presently, laser emission between 500 and 600 nm is desired. A source of impurities or dopants  10  is initially coated on substrate surface  34  to provide a substantially uniform coating of dopants  10  over substrate surface  34 . 
     Pulsed visible laser beam  36  is emitted from visible laser  18  to impinge on the coated dopant  10  formed on substrate surface  34 . 
     Pulsed visible laser beam  36  is shown to irradiate the dopant  10  in predetermined regions which are spatially selective on substrate surface  34 . The dopant regions being irradiated by visible laser  18  are shown in  FIG. 2  as elements  10   a ,  10   b  and  10   c . These dopant regions shown in  FIG. 2  are for illustrative purposes only and in practice, such dopant regions  10   a ,  10   b  and  10   c  would be formed in some selective pattern on substrate surface  34 . 
     Substrate  12  may be formed of a variety of materials, however, in common practice such is generally a silicon substrate and the substrate  12  may be in some cases extremely thin to provide flexible electronic structures which would be used when polymer compositions are used as a material for the substrate  12 . 
     Dopant  10  may be deposited on substrate surface  30  through a variety of techniques: Plasma Enhanced Chemical Vapor Deposition (PECVD), spinning the dopant onto the substrate surface  34  to form some type of liquid organic matrix, or through the formation of a spin-on glass. 
     The subject system and method uses a technique which permits spatially selective laser assisted doping to irradiate only those areas of substrate surface  34  where doping is required in some predetermined pattern. Thus, through use of the present method, only those areas of substrate  12  and substrate surface  34  are irradiated which will include some particular semiconductor element. 
     The subject method in particular uses a visible laser  18  which produces a pulsed laser beam  36  in the visible portion of the spectrum and preferably within the wavelengths between 500-600 nm. This range of visible light is preferentially absorbed by silicon substrates, which is in contrast to UV lasers used in conventional doping methods, having an upper limit of about 400 nm. The absorption coefficient of silicon is ˜10 4 -10 5  cm −1  for λ≈550 nm. The preferential absorption of the visible pulsed laser beam reduces the surface heat buildup exhibited by materials such as silicon dioxide or polymers which are irradiated by conventional UV light. 
     In general, the visible pulsed laser beams  36  used (pulsed copper vapor lasers for instance) are highly incoherent and have a large beam quality or beam propagation factor M 2 , which is essentially &gt;15. The beam quality factor is well known in the art. Its exact mathematical description depends on how the laser beam width is defined. The width of an arbitrary laser beam can be quantified according to the “D86” diameter, knife edge percentages of integrated intensities, or 1/e and 1/e 2  intensity points, among other methods. Regardless of the chosen definition, M 2  gives a measure of the “quality” of an arbitrary beam, relative to a lowest order, single mode Gaussian or “ideal” beam. Equivalently, the value of M 2  indicates the number of “times diffraction limited” factor for the actual beam in the transverse direction of the beam spot. Additional considerations such as the chosen laser source and energy absorption characteristics of the substrate are manifested by selecting laser pulse repetition rates not exceeding 50 Khz, and maintaining individual pulse widths&lt;approximately 200 ns. 
     Referring now to  FIG. 3  there is shown a method utilizing homogenizing optics  20  in combination with focusing optics  24  to produce a substantially uniform beam applied to dopant selective regions  10   a ,  10   b  and  10   c . As clearly shown in  FIG. 3 , visible laser  18  is displaced from substrate  12  and surface  34  wherein focusing optics  24  and homogenizing optics  20  are interspersed between laser  18  and substrate  12 . Homogenizing optics  20  are particularly used to overlap and average the spatial modes of laser beam  36 . Such homogenizing optics may be a glass rod  20  or other suitable optical component well known in the art and commercially available which may be used for homogenization. A more spatially uniform beam  38  passes from glass rod or other homogenizing optics  20  and provides for an increased spatially uniform beam  38  wherein the beam  38  has a substantially smaller beam quality factor than beam  36 . This allows for concentration of more of the laser energy into lower order spatial modes. The spatial selectivity, beam homogenization and energy-stable delivery permit fewer laser shots necessary to drive dopant  10  into substrate  12  in the allotted regions  10   a ,  10   b  and  10   c . Thus, the total number of pulses required to complete implantation may be reduced by almost an order of magnitude using the present technique. 
     As further shown in  FIG. 3 , aperture  22  may be formed in the path of beam  38  to transform the beam into a particular contour such as a rectilinear shaping. The choice of beam shape depends in part on the particular array or plurality of predetermined regions that are to be irradiated. The rectilinear shape is particularly useful for overlapping two or more regions at the same time. 
     Focusing optics  24  may be inserted between homogenizing optics  20  and substrate  12  (subsequent to passage through the aperture  22 ) to focus the laser beam onto the particular dopant areas  10   a ,  10   b  and/or  10   c  formed on surface  34  of substrate  12 . The focused laser energy then drives the dopants  10  into the substrate for activation or formation of a desired semiconductor element. 
     Thus, the method as shown in  FIG. 3  includes depositing a source of dopants  10  onto surface  34  of substrate  12  for defining a multiplicity of selective doping regions  10   a ,  10   b  and  10   c . Visible laser  18  establishes a laser source to emit a laser beam  36 . The laser beam of visible laser  18  is ultimately focused in selective areas of substrate surface  34 . A multiplicity of spatial modes of the laser beam is homogenized to produce a substantially spatially uniform laser beam  38  through the use of homogenizing optics  20  and the laser beam is focused subsequent to spatial uniformity being achieved, onto either dopant regions  10   a ,  10   b  or  10   c  defining the selectively predetermined regions for implanting the dopants into the surface of the substrate  12 . 
     An aspect of controlling and positioning the beam with respect to the substrate surface is illustrated  FIG. 4 . A fast scanning, 2-axis galvanometer  26  irradiates the plurality of predetermined doping regions  10   a ,  10   b , or  10   c  of the substrate  12 . The 2-axis galvanometer allows rapid scanning of the beam in two dimensions across the substrate surface. The rapid scanning capability in essence provides for multiple “beamlets” to irradiate the predetermined doping regions. Generally, a galvanometer consists of two motor-controlled rotating mirrors, each reflecting the laser beam in a different dimension (x/y). The galvanometer  26  is disposed between the homogenization optics  20  and the substrate surface  34 , scanning the homogenized beam  38 . Additionally, the 2-axis galvanometer  26  may also include a combination of optics for focusing the homogenized laser beam  38  during scans across the substrate surface  34 . 
     The aperture  22  provides a rectilinear shape or contour to laser beam  36 . The 2-axis galvanometer then rapidly scans the rectilinear beam across the surface  34 . In this embodiment the area of the beam that is scanned is slightly larger than the area of the predetermined dopant region. 
     In some instances the galvanometer may also position the focused beam longitudinally with respect to the substrate, along the laser beam propagation axis. To ensure proper absorption and implantation of dopants into the substrate, the surface should be planar: flat to within about +/−30% of the Rayleigh range of the focused laser beam  38 . The Rayleigh range, given by π·w 0   2 /λ, where w 0  is the beam&#39;s spot size at the laser beam waist, and is the beam&#39;s wavelength, is a measure of the length of the waist region along the longitudinal direction or beam propagation direction. 
     Control of the homogenized and focused beam may also be provided by a 3-axis stage: transversely across the substrate surface, and longitudinally along an axis perpendicular to the surface.  FIG. 5  schematically shows a 3-axis stage  28  providing and controlling relative movement between the laser  18  and the substrate  12 . 3-axis stages are well known in the art, each stage of the integral unit controlling a separate dimension of movement (up/down, forward/backward, left/right, etc.). The 3-axis stage  28  is disposed between the substrate  12  and the combination of laser  18 , homogenization optics  20 , and focusing optics  24 . The 3-axis stage permits the beam  38  to be scanned across desired doping regions  10   a ,  10   b , or  10   c  on the substrate surface  34 , and can also move the focus location along the beam&#39;s axis, further into the substrate  12  if desired. Both 2-axis galvanometers and 3-axis stages allow either the position of the beam focus to be moved with respect to a stationary substrate or vice versa, and in both instances, selectively predefined regions are irradiated. 
     Referring now to  FIG. 6  there is shown a plurality of predetermined areas of the substrate irradiated by a plurality of beams or “beamlets”, wherein the plurality of beams is provided by a predetermined array of light pipes. As the name indicates, light pipes comprise any number of “pipe-like” delivery mechanisms for the laser light. In this particular embodiment a fiber bundle is chosen. The bundle functions as a 1×N delivery system: there is one input and N outputs, each containing a “beamlet” of laser energy. The incident laser beam  36  illuminates an input face of a fiber bundle  30 . The bundle consists of multiple fiber optic paths through which the incident beam(s) travels. In this fashion, multiple beams or “beamlets”  30   a ,  30   b , and  30   c  are produced and utilized for simultaneously irradiating a plurality of dopant regions  10   a ,  10   b , or  10   c  of the substrate surface  34 . These dopant regions correspond in number and position to each of the individual fiber bundles. The fiber bundles help to further homogenize the incident beam. In this fashion, the entire plurality of predetermined dopant regions is simultaneously and rapidly irradiated. Focusing optics  24  may be disposed after the “beamlets” exit the bundles. 
     Depicted in  FIG. 7  is another embodiment of the subject method, in which the laser beam  36  is directed through a holographic optical element  32  in order to produce the plurality of “beamlets”  32   a ,  32   b , or  32   c  (other optical elements not shown). Holographic optical elements (HOE) are a class of diffractive devices. They can be used as lenses, mirrors, gratings, etc. Among their advantages are ease of manufacture, low weight, and they can be formed in very thin films. They are wavelength dependent and their small size complements existing methods and systems where space is a critical factor. In the subject method, the HOE breaks up the homogenized laser beam  38  into the multiple “beamlets”  32   a ,  32   b , or  32   c  and also focuses these “beamlets” to the predetermined dopant regions  10   a ,  10   b , or  10   c.    
     It will be apparent to those skilled in the art that further combinations for providing a plurality of beams, and as well as homogenizing and focusing optics can be utilized while remaining within the scope of the method of the present invention 
     The disclosed method provides a unique and novel way of introducing dopants into a substrate for fabricating a semiconductor material. The problems of excess surface heat build-up and excess laser energy attributable to conventional UV lasers are solved by using spatially selective doping by pulsed, visible lasers. Only predetermined areas of the semiconductor substrate that are required to contain desired elements need to be irradiated. Hence, the laser irradiates only those areas, thus reducing overall exposure time as well as reducing the amount of unnecessarily deposited laser energy. Overlapping and averaging the spatial modes of the incoherent beam provides a more energy-stable, uniform beam with superior energy deposition characteristics. This reduces the number of laser shots required in conventional doping methods, further saving time and energy costs. 
     The described embodiments are presented for clarity and are exemplary; they are not limited to such. Further enhancements and modifications of the subject method can be achieved without departing from the spirit and scope of the invention.