Abstract:
A method of manufacturing a waveguide within a substrate by local modification of material structure under high power density laser radiation applied from the mostly distant side of the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is being filed under 37 U.S.C. 111 as a continuation application of International Application Number PCT/IL2011/000041 which has an international filing date of Jan. 13, 2011 and which claims priority to the patent application that was filed on Jan. 20, 2010 in Israel and assigned serial number 203408. The above-identified international application is presently pending at the filing of this application and includes at least one common inventor. This application claims the benefit of the priority date under 35 U.S.C. 120 of International Application Number PCT/IL2011/000041 which has an international filing date of Jan. 13 2011 and the Israeli patent application that was filed on Jan. 20, 2010 and assigned serial number 203408. This application incorporates the above-identified applications by reference in their entirety. 
     
    
     TECHNOLOGY FIELD 
       [0002]    The present apparatus and method relate to the field of bulk solid materials processing by ultra-fast laser exposure and more specifically to laser inscription within silicon wafers for example for creating in the wafer internal waveguides. 
       BACKGROUND 
       [0003]    The present invention relates to a field of processing bulk solid materials by help of ultra-fast laser exposure and more specifically for laser inscription inside silicon wafers, for example for creating internal waveguides. 
         [0004]    Recent progress in generating ultra-fast pulse lasers has opened new material processing opportunities usually termed as micro-machining For example, it is now possible to modify material properties inside bulk materials using lasers with wavelength for which the processed materials are transparent. This is primarily due to multi-photon absorption and some other related phenomena which take place in transparent materials when light or radiation power density conveyed into the material exceeds a certain threshold. Such high power threshold is achievable by using high-power ultra-fast lasers by concentrating the emitted radiation energy into very short pulses in the range of femtoseconds to nanoseconds. 
         [0005]    Most publications (see the attached list) and practical implementations relate to such transparent optical materials as glass, silica, Lithium Niobate, and some other materials. Processing of semiconductor materials such as for example, silicon having different types of crystalline structure has been much less investigated and implemented. 
         [0006]    Recently it was shown that within silicon wafers that are usually used for manufacturing electronic devices it is possible to create an optical high frequency modulator. This enables creation of a new field of electro-optical devices and applications, which can be fully implemented within silicon chip. Development of this capability requires availability of a technique supporting formation of different optical paths and schemes for guiding optical beams within bulk silicon, which currently does not exists. One possible way of enabling controlled optical beam propagation within solid (bulk) material is to create a waveguide having desirable location, size and shape. Nejadmalayeri et al disclose an attempt to create optical waveguides in a bulk silicon wafer, relatively deep from the wafer surface. Nejadmalayeri used femtosecond laser pulses in IR spectral range with power sufficient to modify silicon structure and thus create channels of a predetermined shape with index of refraction different from the surrounding substrate. However, it was realized that “waveguides appeared at only very small distances of approximately 5-20 um below the silica-silicon interface, irrespective of the laser focusing depth (0-370 micrometer from the silica-silicon interface)”. 
         [0007]    Accordingly, the problem of formation of different optical paths and schemes for guiding optical beams within bulk silicon as well as a method of modification of a transparent material by laser processing relatively deep from the surface of a silicon substrate remains not solved. 
       GLOSSARY 
       [0008]    In the context of the present disclosure the term “damage” represents any modification of an initial crystallographic structure such as crystallographic structure changes, structure amorphization, inducing in the substrate cracks, voids and the like. Such localized and shaped damaged region may serve as a waveguide, as an electrical isolation, as a center of impurity gettering, and may also be the basis for the creation of new solid state devices or improvement of existing ones. 
       BRIEF SUMMARY 
       [0009]    One aspect of the current method is based on forming a local continuously modified (e.g. damaged) region below the substrate front surface. The modified region or layer may be of an arbitrary 3D structure, a continuously modified layer and a partially modified layer, e.g. a matrix of modified material islands or volumes within the same layer 
         [0010]    The present method includes exposure of a selected region of a substrate using ultra-short pulse focused laser beam with a wavelength at which the processed material is transparent. The laser beam is directed and focused into the substrate from the surface most distant relative to the desired location of the modified region. Such method of the laser beam focusing forms on the first illuminated surface of the substrate a relatively large spot with relatively low power density preventing formation of defects near the exposed surface. 
         [0011]    It is another aspect of the current method to further minimize laser beam power density on the first exposed surface, by coating the surface by an anti-reflection coating (ARC) optimized for the used laser wavelength. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic illustration of a waveguide example produced in depth of a silicon substrate; 
           [0013]      FIGS. 2 and 3  are schematic illustration of a laser beam interaction with a silicon substrate by its illumination from a front and back substrate&#39;s sides, respectively; 
           [0014]      FIG. 4  illustrates an interaction of a laser beam with the silicon substrate having an anti-reflection coating; and 
           [0015]      FIGS. 5A and 5B  (collectively referred to as  FIG. 5 ) illustrate focusing of a laser beam within the silicon substrate using a reflective objective, which includes a mechanism for controlling aberrations. 
       
    
    
     DESCRIPTION 
       [0016]      FIG. 1  is a schematic illustration of a waveguide example produced in depth of a silicon substrate. It schematically shows a single crystal silicon substrate  100  having a front surface  104  and a back surface  108 . A waveguide  112  having a first end  116  and a second end  120  is produced in the substrate. Waveguide  112  is a three dimensional structure (3D) with refractive index different from the surrounding material. Waveguide  112  includes segments  124  and  128  shown as being parallel to substrate  100  surfaces, but their respective axes  132  and  136  may be located at different depths with respect to the surfaces of substrate  100 . Axis  132  is located at a depth  140  and axis  136  is located at a depth  144 . Although shown as located in the same plane (drawing plane), axes  132  and  136  may be located in different planes, oriented at an angle to each other, as it may be required by the desired waveguide (optical) path. It should be noted that any other materials may be used as the substrate and the materials may be of different crystalline structure e.g. polycrystalline, multi-crystalline, micromorphous and amorphous. Spatial (3D) location, dimensions and shape of the waveguide region and segments may be different (For example, a complete continuous layer spread over the whole substrate and coplanar to one of the surfaces or a partially modified layer, e.g. a matrix of modified material islands or volumes located within the same layer may be formed.) but in all of the variations the region of the waveguide is characterized by a different material structure relative to the surrounding region of substrate  100 . 
         [0017]      FIG. 2  schematically illustrates a known method of creating a different structure (a waveguide) in a pre-defined region within the substrate  200  having a front surface  204  and back surface  208 . A beam of optical radiation  212 , which may be a laser radiation beam, is focused by a lens  216  in plane  220 , located at depth  224  below front surface  204  of substrate  200 . In order to penetrate substrate  200  to a desired depth, spectral range, or at least a single wavelength of the used radiation, may be selected such that the substrate material would be substantially transparent at this wavelength or of the particular spectral range. In the vicinity of lens  216  focal plane  220 , and in particular at the depth  224 , the radiation density could be sufficiently high for causing modification of the substrate structure and form a volume  228  with a different material structure. It should be noted that focused laser radiation beam  212 , propagating along an optical axis  232  causes increased radiation power density about the focal plane. If the focal plane is located below the first surface or front surface  204  of substrate  200 , but relatively close to it, the power density at the front surface may be sufficiently high for causing surface damages within the surface area  236  and at a depth proximate or almost coinciding with the surface  204 . This is because the surface layer of the substrate usually contains many structural defects, which could be centers absorbing the applied optical radiation and promoting further material damage generation. Therefore, a combination of high power density near the substrate surface and surface defects may create the damaged region at a shorter than the desired distance (shallower depth) from the substrate surface. 
         [0018]      FIG. 3  schematically illustrates generation of a damaged region according to the present method. The damaged region could be formed within substrate  200  and may serve as a waveguide. In order to significantly reduce the probability of creating a damaged region proximate to the first surface  204 , as it may happen in the above described method, the laser radiation  312  is applied from the back surface  208  of substrate  200 , which is located most distantly from said trajectory first end.  FIG. 3  illustrates that a distance  316  of pre-determined plane  220  from the “first” illuminated surface, which is now surface  208 , is substantially larger than from the front surface  204 . Lens  320  focuses radiation  312  at the focal plane  220  creating within depth  316  of substrate  200  a power density sufficient for modification of substrate  200  material structure. The modification of the substrate may be such that a trajectory defining a continuous region structure will be modified, a complete layer of material spread all over the substrate coplanar with one of the surfaces will be modified or a partially modified layer, e.g. a matrix of modified islands or volumes located within the layer. Since the distance from the first illuminated surface  208  to the focal plane  220  is relatively large, the illuminated by laser radiation spot  324  on surface  208  is also large and the power density within the spot is relatively low. Such low power density prevents formation of defects at surface  208  or in layers/volumes of substrate  200  proximate to the surface. 
         [0019]    It should be noted that  FIGS. 2 and 3  as well as  FIG. 4  are schematic figures and the path of optical rays does not account for changes in direction of the rays, which according to the Snell law occur when optical radiation propagates from a medium with one refractive index into a medium with a different refractive index. 
         [0020]      FIG. 4  schematically illustrates another example of a method of creating a damaged region according to the present method. In order to reduce the laser&#39;s power as well as probability of potential damage to the “first” surface  208  of substrate  200 , surface  208  is coated by a thin transparent film  404  serving as an anti-reflection coating (ARC). In the case of silicon substrate the coating may be such as silicon nitride, which has an index of refraction close to a root square of the index of refraction of silicon. Such condition ensures adequate ARC performance. Typically, optical thickness, which is a multiple of geometrical thickness of the film and index of refraction of the film, would be a quarter of the selected wavelength of the laser radiation. Such ARC can be produced by several manufacturing methods like chemical vapor deposition (CVD), vacuum physical deposition (sputtering), and others. 
         [0021]    The ARC enables to significantly reduce the power of the laser source; in a case of a silicon substrate the ARC reduces the surface reflectance from about 40% to about zero percent. Because of this, the required laser power may be reduced on a similar factor as compared to laser power required to induce a damage in the substrate without the ARC. The lower power of the laser source reduces the radiation power density at the first illuminated surface and therefore reduces probability of causing or developing undesired defects near this surface. In addition to this, use of a lower power laser reduces the cost of the apparatus for making waveguides in the substrate. 
         [0022]    In the examples illustrated in  FIG. 3  and  FIG. 4  the substrate is a silicon wafer with thickness of 0.5 mm to 6.0 mm and the desired depth of the damaged region is in the range of 50 micrometer to 5 mm from the front surface. In particular tests the selected inscription plane was located about 200 um below the surface of the substrate which is first illuminated by the radiation or front surface. 
         [0023]    The produced by the present method waveguide structure typically had a “tube-like cross section” with about 10 micrometer diameter centered around the axis of the tube, located in the plane parallel to the substrate surface. Laser wavelength selection criteria were generally accounting for silicon transparency characteristics, which are known to be transparent for wavelengths larger than 1.1 micrometer. Laser radiation power selection criteria were based on selection of power sufficient to create damages in silicon thereby modifying silicon structure and generation of power density sufficient for creating a multi-photon absorption, which is known to be in the range of 1 mJ/cm 2  to 10 J/cm 2 . 
         [0024]    The required laser power density was produced by focusing laser radiation in a very small volume, e.g. a volume of about several micrometers in diameter, during a short pulse, e.g. in the range from several femto-seconds to a few nano-seconds. 
         [0025]    Typical set-up parameters were: 
         [0026]    Erbium (Er) Fiber Laser Smart Light MD10, commercially available from Raydiance, Inc., Petaluma Calif. U.S.A., with the following specified characteristics:
       Laser Wavelength: 1552 nm   Energy per pulse: 10 μJ   Pulse width (typical): 800 fs   Repetition Rate: 1 Hz-300 kHz   Average Power: 3 Watts   Beam Diameter: 4 mm   Beam Divergence: 1 milliradian       
 
         [0034]    The emitted beam was focused into a spot of about 10 micrometer diameter providing in a single pulse a power density of about 10 J/cm2, which proved to be sufficient for generating two-photon absorption in the irradiated volume. 
         [0035]    The parameters of other set-up elements shown in  FIG. 3  and  FIG. 4  relate mainly to the laser beam focusing lens  216  and  316 . This objective lens was selected based on the following criteria, although other criteria may be employed in selecting a similar lens: 
         [0036]    High numerical aperture (NA) ensuring minimal depth of focus around the focal plane; 
         [0037]    Large working distance allowing moving the focal plane within the substrate in a relatively large range, e.g. up to several millimeters from the front surface in the depth of the substrate; 
         [0038]    High resolution, ensuring minimum aberrations and providing maximal power density of the focused laser radiation within the processed substrate volume; 
         [0039]    Aberration compensation mechanism, which minimizes aberrations as focusing depth changes; 
         [0040]    High transmittance at the selected wavelength, i.e. 1552 nm minimizing losses of laser radiation power; 
         [0041]    High damage threshold although supporting safe operation with high power, short pulse laser radiation. 
         [0042]    An off-the-shelf reflective Schwarzschild objective #506-120 commercially available from Davin Optronics Ltd. (Watford-Hertz, UK) [www.davincatalogue.com] met most of the above criteria. Below are listed the parameters of the objective lens:
       NA: 0.65   Focal length: 3.55 mm   Working distance: 1 9 mm   Small minor diam.: 4 6 mm   Reflectance: 98% (gold coating):
 
The objective includes an adjustment mechanism operative to compensate for the optical aberrations induced by the additional optical thickness within the working distance.
       
 
         [0048]    An apparatus for generating waveguides in a substrate according to the present method and employing the above components is schematically illustrated in  FIGS. 5A and 5B , where two different distances ( 532  and  544 ) of the focal plane relative to the first substrate surface are shown. 
         [0049]    A substrate  500 , which may be a silicon wafer or any other suitable substrate has a first or front surface  504 , which may be either the front or the back surface, and the second or back surface  508 . Laser beam  512  is focused by a reflective objective  516 , for example such as Schwarzschild objective #506-120. The objective includes a first minor  520  and a second minor  524 . In  FIG. 5A , Focal plane  528  of objective  516  is located at a distance  532  from the front illuminated surface  504 . Laser beam  512  enters objective  516  through first mirror  520  aperture  536  and it impinges on second minor  524 , which reflects the incident beam into a large angular range. All light beams reflected by second minor  524  are then reflected by first mirror  520  and focused into a small spot  540  in the focal plane  528 . 
         [0050]    The spot size is limited by diffraction, i.e. by the NA of the objective and laser wavelength, as well as by aberrations of the optical system. The major contributor to the optical aberration is the silicon layer through thickness  532  of which light beam  512  has to propagate until it reaches focal plane  528  where the waveguide is to be formed. In order to minimize this aberration, the distance  548  between the first and the second mirrors of objective  516  may be adjusted to an optimal value. 
         [0051]    The waveguides may be created within the substrate in one plane or in multiple planes. The waveguides may be created to affect the desired segments/volumes of a plane/layer or even a complete layer spread all over the substrate. In some cases the desired waveguide pattern may be located at different depths within the substrate, which may be a silicon wafer or other substrate.  FIG. 5B  illustrates objective  516  refocused to affect substrate  500  at a depth  544  different from depth  532  ( FIG. 5A ). In order to keep the minimal spot size in the focal plane and compensate for aberrations that may be caused by the change in thickness of the substrate layer, the distance between first  520  and second  524  mirrors of the objective will be adjusted accordingly as shown by distance  560 . 
         [0052]    The refractive index of the substrate is higher than that of the air and according to the Snell law oblique incidence rays, such as rays  552 , are refracted by the substrate and propagate at smaller, as measured to the perpendicular to the surface, angles, shown by the marginal ray  556 . The higher index of refraction of the substrate, the stronger the refraction. The stronger the refraction, the higher the power density of the incident illumination at the substrate surface. In materials with relatively low index of refraction such as glass or silica this effect is small. However, in materials with high index of refraction like silicon this effect is very strong. Therefore generating near-surface defects with laser illumination is more likely to happen in silicon than in silica. 
         [0053]    The described approach of processing substrates can be used for modification production of the desired/required trajectories defining a continuous region structure or separate volumes of waveguides by providing a relative displacement between the substrate and the focused spot. This may be achieved either by moving the substrate or by moving the optics or by a combined movement of both the substrate and the optics. The velocity of such movement depends on the power of the optical radiation provided by the laser, which in case of a pulsed laser depends on the repetition rate of the laser. A certain overlap between adjacent laser spots selected to provide a continuous modified by laser radiation material structure (line or plane) may be provided. The distance of the focal plane relative to the first illuminated surface may also be changed in course of desired waveguide trajectory formation. Such change of distance will be synchronized with simultaneous compensation for the induced aberrations caused by the change in the focal plane location. 
         [0054]    One of the potential applications of the method described is production of photonics devices, for example formation of waveguides in silicon. Other possible applications are in areas such as microelectronics and photo-voltaic solar cells manufacturing, wafers marking and others. 
         [0055]    Although the method and apparatus implementing the method have been described in conjunction with specific examples thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the description is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims: