Beam or wave front

An improved beam for use in ablating a surface to be processed. The improved beam front has a profile with a plurality of undulation and the plurality of undulations define a plurality of annular peaks and annular valleys. An intensity of each annular peak is about 10 percent greater than an intensity of each annular valley whereby the undulation of the improved beam facilitates formation of a substantially uniform feature in the surface to be processed. A method of utilizing an improved beam to form the desired feature in a surface to be processed is also disclosed. To reduce energy losses, a fourier transform is performed on the supplied laser beam. Before striking the surface, this beam passes through a focusing lens which performs a reverse fourier transform and retransforms the wave front substantially back to its originally wave form and intensity.

FIELD OF THE INVENTION

The present invention relates to an improved beam or wave front to facilitate a more uniform and concentric drilling or ablation in a surface of a substrate or object to be processed.

BACKGROUND OF THE INVENTION

In the prior art, there have been attempts to achieve a substantially flat top or pseudo flat top wave front or beam for use in a drilling or otherwise forming a blind via, hole, an aperture or other desired formation in a surface to be processed. One such pseudo flat top wave front or beam is generally shown inFIG. 1of the drawings and this type of beam generally, when striking a surface to be processed, forms a blind via or aperture therein which has a shape generally depicted inFIG. 2of the drawings. The problem with this type of blind via or aperture is that a depth of the blind via or aperture tends to be deeper around the perimeter of the via than it is in the center. This non uniformity results in a blind via or aperture which may be unacceptable for some manufacturing operations or processes.

The inventor believes that one of the problems associated with flat top or pseudo flat top wave fronts or beams is that edge diffraction occurs as the wave front or beam strikes or contacts the surface to be processed. This edge diffraction has a tendency to increase the drilling depth of the blind via or aperture around the perimeter thereof and this results in a blind via or aperture which has an undesirable contour or profile. As can be seen inFIG. 1, a well known laser beam2, with a fat top or pseudo flat top wave front4, is diagrammatically depicted. This type of laser beam2has an intensity “I” and a diameter “D” and is typically used for drilling and forming holes in objects. However, one major problem with the laser beam2, when drilling a blind hole in a desired object, for example, is the formation of an uneven base surface6at the bottom of the blind via8. This uneven base surface6is generally shown in FIG.2.

The perimeter region10of the blind via8is cut significantly deeper than a central region12of the blind via8thus creating the uneven base surface6. This defect may increase manufacturing time, result in an improper fit of a desired component, or even possible rejection or discarding the object14. All of these scenarios ultimately increase the associated manufacturing and assembly costs and are to be avoided. Therefore, the flat top wave front4is generally unacceptable for many manufacturing applications, especially when close tolerances are required.

SUMMARY OF THE INVENTION

Wherefore, it is an object of the present invention to overcome the above mentioned shortcomings and drawbacks associated with the prior art wave fronts or beams.

Another object of the present invention is to provide the beam with a profile which minimized the edge diffraction effect to result in the drilling or formation of a more uniform blind via, hole, aperture or some other desired formation in a surface to be processed.

A further object of the present invention is to accentuate the intensity of the beam or wave front about its perimeter, especially when drilling through metal surfaces, to facilitate a precise and uniform blind via, hole, aperture or some other desired formation in a surface to be processed while minimizing the amount of annealing, or some other undesired side effect, which may occur as a result of such drilling.

A further object of the present invention is to provide a more uniform plasma, as the laser beam strikes the surface of the object to be processed and to provide a more average extenuation of the plasma resisting the supply laser beam and this facilitates a more uniformly drilled or formed aperture or feature formed in the surface to be processed.

Yet another object of the present invention is to vary the intensity of the beam or wave front, from a center longitudinal axis of the beam or wave front radially outward toward the perimeter of the beam or wave front, to form a plurality of annular peaks and annular valleys which facilitate a more uniform formation of a blind via, hole, aperture or some other desired formation in a surface to be processed.

A common problem that occurs with flat and pseudo flat laser beams or wave fronts is that when the beam strikes the surface of the object to be drilled and commences formation of the blind via, hole, aperture or other formation therein, a plasma is generated which typically emanates from the central portion of the formed hole, aperture or other feature being formed in the surface of the object to be processed. This plasma emanates from the central area of the hole, aperture or other feature being formed in the surface and is believed to attenuate the central portion of the laser beam thereby decreasing the depth of the central area of the hole, aperture or other feature being formed in the surface but not diminishing the perimeter portion of the hole, aperture or other feature being formed in the surface. The non attenuated beam tends to result in a deeper drilled depth around the circumference or perimeter of the hole, aperture or other formation while the attenuated beam tends to result in a shallower drilled depth in the central portion of the hole aperture or formation. The inventor has found that by modifying the wave front so that it has a plurality of annular peaks and annular valleys, such modification results in a more uniform plasma so that the attenuation by the plasma is experienced more uniformly across the entire beam or wave front and this results in a drilled hole, aperture or other feature in the surface of the object to be processed having a more uniformly and consistently drilled depth for the entire hole, aperture or other formation in the surface.

The present invention also relates to an improved beam for use in ablating a surface of an object to be processed, the improved beam front having a profile with a plurality of undulations, the plurality of undulations defining a plurality of annular peaks and annular valleys, and an intensity of the annular peaks being about 10 percent greater than an intensity of the annular valleys whereby the undulations of the improved beam facilitating formation of the uniform feature in the surface to be processed.

The present invention also relates to a method of utilizing an improved beam to form the desired feature in a surface to be processed, the method comprising the steps of: emitting a beam from a laser; shaping the emitted beam to have a profile with a plurality of undulations, the plurality of undulations defining a plurality of annular peaks and annular valleys, and an intensity of the annular peaks being about 10 percent greater than an intensity of the annular valleys; and contacting a surface to be processed with the improved beam so that the undulations of the improved beam facilitating formation of a uniform feature in a surface to be processed.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of an improved laser beam22, according to the present invention, is shown in FIG.3. The inventor has determined that by altering the shape or profile of the wave front24of the laser beam22so that the wave front24has a plurality of undulations comprising a plurality of alternating annular peaks28and annular valleys30which extend radially from a center, or central longitudinal axis, of the beam22. The improved laser beam22, of the present invention, when contacting or striking a surface to be processed32, facilitates a more uniform blind via, hole, aperture or other formation34in the surface of the object to be processed32. If the blind via, hole, aperture or some other formation34in the surface of the object to be processed32can be adequately formed by a single or minimal amount of firings of the laser, this reduces the manufacturing time in order to obtain an acceptable blind via, hole, aperture or some other formation34in the surface of the object to be processed32. This, in turn, increases the throughput of the manufacturing equipment utilizing the improved laser beam22according to the present invention. Preferably, the annular peaks28will have an intensity of at least about 10 percent greater than an intensity of the annular valleys30of the laser beam22and the wave front24will have a plurality of annular peaks28and annular valleys30forming the laser beam22. Preferably, the diameter of the laser beam will taper toward the leading end of the beam at an angle θ of between 0 and 20 degrees.

FIG. 4depicts a second embodiment of the improved laser beam22according to the present invention. This embodiment is similar to the embodiment inFIG. 3with the addition of an enlarged annular perimeter peak36circumscribing the entire perimeter of the wave front24. The enlarged annular perimeter peak36is approximately 10-30 percent or so greater than the intensity of the annular valleys30. This embodiment is particularly advantageous when drilling into an object32which comprise multiple layers of different materials, as can be generally seen inFIGS. 9A-9C. A further description concerning the same will follow below.

A third embodiment of the improved laser beam22, according to the present invention, is shown in FIG.5. This embodiment is similar to the embodiment ofFIG. 4with the addition of an enlarged central peak38. Enlarged central peak38further assists with an initial first cut into an object32prior to the remainder of the wave front24contacting the object to be processed32. The enlarged annular perimeter peak36and the enlarged control peaks38both have an intensity which is approximately between 10 and 30 percent or so greater than the intensity of the annular valleys30, and more preferably about 15 percent or so greater than the intensity than the annular valleys30.

With reference toFIG. 6, a fourth embodiment of the improved laser beam22, according to the present invention, can be seen. According to this embodiment, laser beam22has a central peak38having a first intensity and annular perimeter peak having a second intensity with the first intensity being approximately between 10 and 50 percent or so greater than the intensity of the annular perimeter peak36, and more preferably about 30 percent or so greater than the intensity of the annular perimeter peak36. The diameter DC of the central peak38is approximately between 10 and 50 percent or so less than the diameter of the annular perimeter peak36, and more preferably about 30 percent or so less than the diameter DP of the annular perimeter peak36. A smooth transition or step44integrates the central peak38with the annular perimeter peak36. It is to be appreciated that the intensity and/or diameter of the central peak38and of the annular perimeter peak36can be modified, as necessary, as would be apparent to one skilled in the art, to suit the desired application.

A fifth embodiment of the improved laser beam22, according to the present invention, is shown in FIG.7. This embodiment is similar to the embodiment ofFIG. 6but includes a plurality of smooth transitions or steps44to integrate the central peak38with the annular perimeter peak36. According to this embodiment, laser beam22has a central peak38having a first intensity and an annular perimeter peak36having a second intensity with the central peak38being approximately between 120 and 400 percent or so greater than the intensity of the perimeter peak36. In addition, at least one and preferably a plurality of intermediate annular peaks46,48are located between the central peak38and the annular perimeter peak36. Each intermediate annular peak46,48has a diameter DI1, DI2which is approximately between 20 and 150 percent or so greater than the diameter DC of the central peak38. Stated another way, each step can be a desired percentage of the entire intensity of the beam22. A smooth transition or step44integrates each peak36,38,46or48with each adjacent peak36,38,46or48. As with the previous embodiment, it is to be appreciated that the intensity and/or diameter of each peak36,38,46or48can be modified, as necessary, as would be apparent to one skilled in the art, to suit the desired application.

Now with reference toFIGS. 8A-8C, a detailed description concerning use of the laser beam22, according to the present invention, to drill a hole will now be provided. As can be seen in these Figures, a laser beam22is diagrammatically shown approaching a surface49of an object to be processed32(see FIG.8A). Upon contacting the surface of the object to be processed32, the annular peaks28of the laser beam22strike and penetrate the surface49of the object to be processed32and commence formation of the blind via34(see FIG.8B). As the laser beam22penetrates the surface49of the object to be processed32, the material50forming the surface49is ablated and tends to be carried away, from the blind via34being formed, with the exhaust stream52from the laser beam22. This exhaust stream52is believed to attenuate the laser beam22and thereby hinder formation of the blind via34having the desired profile. The improved laser beam22, according to the present invention, overcomes this drawback by compensating for the attention of the exhaust stream52and thereby results in the formation of a blind via34with a flatter and more uniform base surface54(e.g. ±5% variation in the base surface), as can be seen in FIG.8C.

With reference now toFIGS. 9A-9C, formation of a blind via, hole, aperture or some other formation34in an object, having two distinct layers60,62, will now be discussed. When forming the desired blind via, hole, aperture or some other formation34in an object to be processed32which has, for example, a metallic layer60, such as copper, on a top surface thereof, it is desirable to alter the shape or profile of the improved laser beam22so that the intensity of an enlarged perimeter portion36of the wave front24is of an intensity which is at least about 15 percent or so greater than an intensity of the annular valleys30of the laser22. A diagrammatic representation of one embodiment of the improved beam22, according to the present invention, is depicted in FIG.9A.

As the laser beam22strikes or contacts the surface to the object to be processed32(see FIG.9B), the accentuated intensity of the enlarged perimeter portion36of the wave front24strikes the metallic layer60to provide a clean fracture or perimeter opening in the metallic layer60. This facilitates a substantially complete obliteration of the metallic layer60covering the area where the desired blind via, hole, aperture or some other formation34is to be formed in the object to be processed32. The accentuated intensity of the enlarged annular perimeter peak36of the beam or wave front also minimizes the amount of annealing of the metallic layer60which might otherwise occur as a result of using a pseudo or flat top beam of the prior art. In addition, it is believed that the increased intensity of the enlarged annular perimeter peak36generates eddy pockets63of an intense plasma which also acts as a wall that facilitates containing a central plasma. The central plasma then is able to cut out or drill the surface of the second layer62in a uniform manner. Once the laser beam22penetrates through the metallic layer60, the beam or wave front continues ablating or drilling a desired blind via, hole, aperture or other formation34to be formed made in the second layer62of the object to be processed32. Following completion of the drilling process, the object to be processed will have a profile which will be somewhat similar to the profile shown in FIG.9C.

Turning now toFIG. 10, a brief description concerning an embodiment for generating the improved beam or wave front, according to the present invention, will now be briefly discussed. As can be seen in this Figure, a conventional laser64is used to generate a laser beam22, and the laser beam22is directed toward first and second mirrors66,68of first and second repeat positioning devices70,72which are coupled to and controlled by a conventional computer74. Prior to reaching the first mirror66,68, the laser beam22passes through a beam shaping device, such as a diffractive element, a holographic element, etc., generally indicated as element76, which changes the shape and/or profile of the laser beam22, in a conventional manner, from a guassian beam or wave front to a semi-pseudo flat wave front laser beam22, as discussed above. The altered laser beam22is then reflected by first and second mirrors66,68, of first and second repeat positioning devices70,72toward a focusing lens78, such as an F-theta lens. Finally, the focusing lens78focuses the supplied laser beam22at a top surface of the object to be processed32.

Although the inventor is not sure why or how the improved beam or wave front, according to the present invention, provides superior drilling characteristics, the inventor believes that by shaping the beam or wave front to have a plurality of undulations with the annular peaks28and annular valleys30, the variation in intensity of the illumination facilitates the formation of eddy pockets63or areas in the hole, aperture or other formation34to be made and such eddy pockets63or areas facilitate a more uniform ablation and removal of the metallic layer60or lower second substrate layer62. The eddy pockets or areas may tend to create turbulence within the hole, aperture or other formation34and thereby results in an opening having a profile generally as shown in FIG.9B.

One problem which inherently occurs to a wave front as a laser beam passes through an aperture is edge diffraction. With reference toFIG. 11, this “transformation” of the wave front of the laser beam will now be described. The laser beam80, e.g., a guassian beam, is emitted and passes through at least one, and possibly two, HOE beam shapers82. The beam shapers82alter the supplied beam to have generally a pseudo flat top wave front indicated as84. As the altered laser beam86passes though the aperture88, the outer perimeter of the wave front84of the laser beam86is altered to the shape shown in the middle section of FIG.11. That is, the diffracted wave front84′ of the laser beam86′ has a shape very similar to that of an airy disc format. The problem with edge diffraction is that some of the energy of the supplied laser beam86is scattered and lost due to the edge diffraction effect and this decreases the efficiency of the laser beam86. In fact, some of the beam energy does not pass through the aperture88and reflected away from the aperture88.

The diffracted wave front84′ of the laser beam86′ is then directed at and passes through a focusing lens90before contacting or striking the surface to be processed92. As the diffracted wave front84′ of the laser beam86′ passes through the focusing lens90, the focusing lens90provides a reverse fourier transform to the diffracted wave front84′ of the laser beam86′ to retransform the diffracted wave front84′ generally back to its originally wave form shape as shown following passing through the beam shapers82, e.g. its original shape prior passing through the aperture88and having its outer perimeter altered by edge diffraction. This retransformed wave front shape is designated by reference numeral84″. However, the intensity of the transformed and retransformed wave front84″ of the laser beam86″ has less intensity, e.g., possibly 10-50% less intensity due to edge diffraction, than the intensity of the original wave front84of the laser beam86before passing through the aperture88.

With reference now toFIGS. 12A and 12B, a discussion of the airy disc format, formed when passing through a circular aperture, will now be provided. As can be seen in both of these Figures, the centrally located peak92of the laser beam which is coincident or adjacent with a longitudinal axis L of the beam has the greatest intensity. The intensity of the beam diminishes quite radically in a radial direction. As a result of this, a peripheral region of the beam has a second annular peak94with an intensity of about 5-10% or so, circumscribing the longitudinal axis L of the beam. A third annular peak96, having intensity of about 1-5% or so, circumscribes both the central and the second annular peaks of the beam. In general, the intensity of the annular peaks diminishes quite radically the further the annular peaks are spaced from the longitudinal axis L of the beam. It is to be appreciated that various other airy disc format wave fronts can be generated depending upon the shape and size of the aperture through which the beam passes through or the number of apertures illuminated.

One solution to avoiding the edge diffraction effect noted above is to initially start with a square profile wave front98, i.e., a flat top beam, for a laser beam100(seeFIG. 13A) and then perform a fourier transform on that laser beam100via a conventional wave front generator101. The fourier transform of the square profile wave front laser beam is shown in the middle portion of FIG.13A and designated as102. This results in the laser beam102having a wave front104very similar to the airy disc format shown and described inFIGS. 12A and 12Bexcept that the edge diffraction effect, described with reference toFIG. 11above, is substantially minimized so that the intensity of the fourier transformed laser beam102is not significantly diminished from that of its initial intensity of the laser beam100, i.e., no edge diffraction losses occur, and the overall efficient of the fourier transformed laser beam102is substantially retained. The fourier transformed wave front104of the laser beam102is then directed at and passes through a focusing lens106before contacting or striking the surface to be processed108. As the fourier transformed wave front104of the laser beam102passes through the focusing lens106, the focusing lens106provides a reverse fourier transform to the fourier transformed wave front104of the laser beam102to retransform the fourier transformed wave front substantially back to its originally wave form shape, e.g. the laser beam shape prior to the initial fourier transform of the laser beam100, and this wave front shape is formed at the image plane109of the imaging lens106and is designated by reference numeral112. Due to the elimination of edge diffraction losses which occur when the beam passes through an aperture, the intensity of the transformed and retransformed wave front112of the laser beam110has substantially the same intensity as the intensity of the original wave front98of the laser beam100prior to the fourier transform. When the retransformed wave front112of the laser beam110strikes the surface to be processed108, virtually all of the supplied energy is utilized to drill, burn or form a more uniform blind via, hole, aperture or some other desired formation in the surface to be processed.

A second embodiment, for avoiding the edge diffraction effect noted above, is shown inFIG. 13Bfor a guassian wave front. The guassian wave front98′ for a laser beam100′ passes through a conventional wave front generator101where the wave front generator creates a representative fourier transform of a pseudo flat top wave front profile on that laser beam100′. The fourier transform of the guassian wave front laser beam is shown in the middle portion of FIG.13B and designated as102′. This results in the laser beam102′ having a wave front104′ very similar to the airy disc format shown and described inFIGS. 12A and 12Bexcept that the edge diffraction effect, described with reference toFIG. 11above, is substantially minimized so that the intensity of the fourier transformed laser beam102′ is not significantly diminished from that of its initial intensity of the laser beam100′, i.e., no edge diffraction losses occur, and the overall efficient of the fourier transformed laser beam102′ is substantially retained.

The fourier transformed wave front104′ of the laser beam102′ is then directed at and passes through a focusing lens106before contacting or striking the surface to be processed108. As the fourier transformed wave front104′ of the laser beam102′ passes through the focusing lens106, the focusing lens106provides a reverse fourier transform to the fourier transformed wave front104′ of the laser beam102′ to retransform the fourier transformed wave front substantially to a square profile wave front, e.g., a flat top beam or any other desired wave front at the image plane, for a laser beam and this wave front shape is formed at the image plane109of the imaging lens106and is designated by reference numeral112′. Due to the elimination of edge diffraction losses which occur when the beam passes through an aperture, the intensity of the transformed and retransformed wave front112′ of the laser beam110′ has substantially the same intensity as the intensity of the airy disc wave front104′ of the laser beam102′. When the retransformed wave front112′ of the laser beam110′ strikes the surface to be processed108, virtually all of the supplied energy is utilized to drill, burn or form a more uniform blind via, hole, aperture or some other desired formation in the surface to be processed.

A third embodiment, for completely avoiding the edge diffraction effect, is shown in FIG.13C. According to this embodiment, a desired airy disc format wave front, having a wave front104″ very similar to the airy disc format shown and described inFIGS. 12A and 12B, is generated in a conventional manner. As the wave front does not pass through an aperture, no edge diffraction effects occur and the overall efficiency of the system is improved over the embodiments ofFIGS. 13A and 13B.

The generated wave front104″ of the laser beam102″ is then directed at and passes through a focusing lens106before contacting or striking the surface to be processed108. As the generated wave front104″ of the laser beam102″ passes through the focusing lens106, the focusing lens106provides a fourier transform to the generated wave front104″ of the laser beam102″ to transform the wave front substantially to a square profile wave front, i.e., a flat top beam, for a laser beam and this wave front shape is formed at the image plane109of the imaging lens106and is designated by reference numeral112″. Due to the elimination of edge diffraction losses which occur when the beam passes through an aperture, the intensity of the transformed wave front112″ of the laser beam110″ has substantially the same intensity as the intensity of the airy disc wave front104″ of the generated laser beam102″. When the retransformed wave front112″ of the laser beam110″ strikes the surface to be processed108, virtually all of the supplied energy is utilized to drill, burn or form a more uniform blind via, hole, aperture or some other desired formation in the surface to be processed.

Referring now toFIGS. 14A,14B,14C and14D, therein are illustrated embodiments for generating the wavefronts of the present invention. In accordance with the above discussions, it will be recognized that the generation of an improved wavefront laser beam according to the present invention involves the transformation of an input beam having, for example, a flattop or gaussian wavefront, into a beam having an airy disc or similar wavefront, as illustrated, for example, inFIGS. 12A-12Bor13A-13C, and the transformation of that beam into a beam having the final desired wavefront, such as illustrated inFIGS. 3-7.

It will be apparent to those of skill in the arts that these laser beam forming operations in turn involve the shaping, compressing and collimating of the laser beams. As described above, the shaping, compressing and collimating operations are performed without the use of apertures or other elements that result in edge diffraction effects, that is, with the use of non-diffracting elements such as lenses or holographic optical elements (HOEs).

It will also be appreciated by those of skill in the relevant arts that while optical elements such as lenses and HOEs are commonly designed to perform a single operation, such elements may also be readily designed to perform two such operations, so that three such operations may be performed with only two optical elements. For example, a single lens or HOE may be designed and used to both shape and compress a beam, or to collimate and shape a beam. In addition, and with appropriate selection and design of the elements, the elements may be arrange and the operations performed on the beams in virtually any order.

Exemplary embodiments of Wavefront Shapers114comprised of Optical Elements116for generating improved laser wavefronts according to the present invention are illustrated inFIGS. 14A through 14D.

First considering arrangements wherein a Wavefront Shaper114is comprised of three separate Optical Elements116, each performing a single function,FIG. 14Aillustrates the sequential arrangement wherein an Input Beam118passes through the elements in the order Compression Element120—Shaping Element122—Collimating Element124to provide the final Shaped Output Beam126.FIG. 14Bin turn illustrates the functionally equivalent arrangement of Shaping Element122—Compression Element120—Collimating Element124. In will be noted that Collimating Element124is the final element in each of these arrangements because Collimating Element124is most frequently comprised of and performs the function of focusing lens78, as discussed with reference toFIG. 10, and is therefore most conveniently arranged as the final element of Wavefront Shaper114. It must also be noted that Collimating Element124will typically be comprised of a lens, while Shaping Element122and Compression Element120may be comprised of Holographic Optical Elements (HOEs), or a combination of lenses and HOEs.

FIGS. 14C and 14D, in turn, illustrate examples of arrangements of Wavefront Shaper114wherein a single Optical Element116performs two functions, thereby reducing the required number of elements from three to two.FIG. 14C, for example, illustrates an arrangement having a Shaping/Compression Element128followed by a Collimating Element124, whileFIG. 14Dillustrates a Compression Element120followed by a Shaping/Collimating Element130. In these embodiments, and for example, a Shaping/Compression Element128or a Compression Element120may typically be comprised of a HOE. A Collimating Element124or a Shaping/Collimating Element130may, however, be typically comprised of a lens as discussed above, wherein it is shown that a lens such as lens106ofFIGS. 13A and 13Bmay both function as a focusing lens78and may perform a reverse fourier transform of a laser beam wavefront.

It is to be appreciated that the present invention is also useful for illuminating or exposing a feature in a photo resist application or for removing gates, traces, etc., of fine metal or dielectrics used for memory and/or semiconductor fabrication, modification and/or repair. The present invention is also useful for imaging materials used in stereo lithography or formation of three-dimensional structures, such as the creation of micro-electro mechanical systems, etc.